Add prefer-class-fields rule

[FRSgit]

Adds prefer-class-fields rule which turns:

class Foo {
  constructor() {
    this.foo = 'foo';
  }
}

into:

class Foo {
  foo = 'foo';
}

Fixes #314

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IssueHunt Summary

Referenced issues

This pull request has been submitted to:

• #314: Rule proposal: prefer-class-fields

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[fisker]

What should we do if the assignment is unreachable?

			class Foo {
				constructor(x) {
					if (x) {return}
					this.foo = 'foo';
				}
			}

[fregante]

What should we do if the assignment is unreachable?

It's dynamically set, so it cannot be statically defined. It should not be reported by this rule.

[FRSgit]

Hey @fregante!
Thanks for your input about the linting - is there a ruleset somewhere which I could use? The lint task in this repo is not doing this much (at least for me) and it removes some of your changes 😄 E.g. it seems that the part /** @type {import('eslint').Rule.RuleModule} */ is required by the lint task - after your change it was failing with:

So, I've reverted this one change.

Or maybe we can just agree that once the work in this PR is done you'll maybe just run your linter on it once?

[FRSgit]

Handled stopping of the static analysis on unsupported case: 3376845

[FRSgit]

Okay, I fixed all test/linting errors connected with my rule - should I fix other as well? They seems to be connected with some default options missing in already existing rules?
cc: @sindresorhus @fregante @fisker

[FRSgit]

Hey! I've merged last changes from main to the branch of this PR - now it's ready to be reviewed again

[sindresorhus]

Change the test file extension from .mjs to .js.

[FRSgit]

@sindresorhus done: 8358a43

I'm not sure yet why the tests are failing - locally they're working just fine.

[FRSgit]

Okay, got it working and all of the requested changes are applied.

BTW it's weird, but for me the npm run fix:eslint-docs script worked only on node@22.
Maybe it would be good to add an .nvmrc file to the root of the repo with the node version that should be used when running npm scripts?

[fisker]

Fixed the fix logic for some edge cases, especially

class Foo {
constructor() {
	this.bar = 1;
}
static}