feat: add lintfix and prettier scripts

[scscgit]

• Add a lint:prettier script to provide users with a recommended way to enforce a unified code style in the project, which is less accessible if they have to research the correct usage, or even access it via npx. The intention should be to provide a configuration where linters don't conflict with each other. Also included this in the lint script by default to enforce best practices (but anyone is free to remove it). I can't comprehend why new users should be forced to research this on their own; we need a reference solution as a source of truth for example if we want to setup IDE extensions and make sure that they converge on the same code style (definitely not trivial with all the options in VSCode like Vetur, Prettier, Eslint, built-in...).
  • As this can run over all files, we need to specify a .prettierignore, especially to exclude directories like .nuxt. Due to Extend .prettierignore with an "include" syntax prettier/prettier#8288 and Proposal: Filter using ".gitignore" first, then ".prettierignore" second prettier/prettier#8506, we have to duplicate the .gitignore content without being able to include it e.g. by referencing two ignore files. Users may require this file to differ from their .gitignore, e.g. if they need to include results of code generators. Note that there is no analogy of eslint-disable (Feature proposal: prettier-ignore-next-line prettier/prettier#9310), so this file cannot be avoided. As for code re-use (DRY) in the template file, I was unable to import the content via EJS, and the SAO generator lacks a copy action: Cannot include EJS partials (second issue), and missing copy action saojs/sao#176
• Add a lintfix script to provide a quick way of fixing any issues in the entire project, especially before they make a commit. Prettier includes --list-different to display only changed files (unified with other linters) instead of every single file in the project.
  • Reorder prettier to the beginning of lintfix to avoid ESLint causing conflicts of rules like no-return-assign (without having to use Prettier via ESlint by installing eslint-plugin-prettier and using plugin:prettier/recommended), fixing fix(prettier): use recommended rules to avoid eslint conflicts, revert #797 #827
• create-nuxt-app: run lintfix at the end instead of hard-coding all of the scripts individually; just like what the user is supposed to do on a regular basis.
• Extend lint-staged in package.json to include prettier by default, where it requires --ignore-unknown Implement a new --ignore-unknown argument for CLI prettier/prettier#8136, as you can reproduce the issue by trying to commit an unstyled file that's referenced in the .prettierignore. Additionally use --cache for eslint as a recommended performance improvement, which is already assumed as our current .gitignore includes the .eslintcache.
• Reordered scripts to have linters near each others, and to keep the prepare at the end. (Or could we move it to the top?)
• Add .ts to supported eslint types. I'd personally prefer keeping it there even if the project is JS-only, but for the sake of consistency I only added it for TS projects.
• Add .scss, .sass, and .html to supported stylelint types. Fixing CSS <style> tags inside HTML definitely sounds like an expected default behavior to me. This list may not be exhaustive, so please make sure it's kept up-to-date with any popular use-cases.
• Modify lintStaged conditions to include it even without eslint. Previously this meant that together with husky they weren't configured if the user has only chosen commitlint, which I believe is a bug. Now it installs even for prettier, but I also allowed it to be installed as an empty block if chosen alone, because the effects of that selection don't make any sense otherwise; who are we to judge if someone wants to lint something else on staging?

[scscgit]

Part of lintfix results thanks to prettier; I think we should integrate it to the template.

[scscgit]

I used EJS templates because there already was <%= pmRun %>, but I don't see a reason not to `use ${variables} instead`; especially as any incorrect templates in this JS cause a compilation error, e.g. if you escape \' within " instead of replacing ' by " and using ' inside. Also the whitespace-trim didn't seem to be very useful here.

[scscgit]

Instead of moving, we could also create it here.

[scscgit]

Here the lint-staged was selected alone, which previously didn't install it.

[scscgit]

I'd also suggest removing the .prettierrc and instead moving its content to package.json (as supported by cosmiconfig) in order to reduce the unnecessary amount of files in the root directory. Most users won't ever touch the defaults, and there doesn't seem to be any advantage in a separate file anyway (e.g. having JS available):

"prettier": {
    "semi": false,
    "singleQuote": true
  },

[scscgit]

Reordered prettier to the beginning of lintfix to fix #827.