Checking promise-related errors

[jrmajor]

Regarding #31 (comment) (cc @benmccann):

Something else I was interested in trying was checking for promise-related errors. I just noticed in another project that TypeScript doesn't catch these types of errors. It should probably be another PR and isn't related to this change at all, but I thought I'd mention it just in case you were interested in working on it at all:

immich-app/immich@7f504ec/web/.eslintrc.cjs#L54

These rules report an error for every file that doesn't have a tsconfig.json. Not only we would need to enable them only for subdirectories that have a tsconfig.json, but we would need to exclude paths that are not included/are excluded in tsconfig.json. Here's how it looks for the main Svelte repo:

{
	files: [
		// only for packages/svelte
		// we would need to specify more paths to make it work for sites/*
		// it needs to be kept in sync with include/exclude paths in tsconfigs
		'packages/svelte/src/**/*',
		'packages/svelte/tests/*/test.ts',
		'packages/svelte/tests/runtime-browser/test-ssr.ts',
		'packages/svelte/tests/*/samples/*/_config.js'
	],
	languageOptions: {
		parserOptions: {
			project: true
		}
	},
	rules: ts.configs.recommendedTypeCheckedOnly[2].rules,
},
{
	rules: {
		// too many errors reported
		// without these, there are 11 errors
		'@typescript-eslint/no-unsafe-argument': 'off',
		'@typescript-eslint/no-unsafe-assignment': 'off',
		'@typescript-eslint/no-unsafe-call': 'off',
		'@typescript-eslint/no-unsafe-member-access': 'off',
		'@typescript-eslint/no-unsafe-return': 'off',
		'@typescript-eslint/unbound-method': 'off'
	}
},

Because of that, we can't just turn these rules on in @sveltejs/eslint-config. We could expose a helper for configuring them, I don't think it's worth it though:

/** @type {import('eslint').Linter.FlatConfig[]} */
export default [
	...svelteConfig,
	...svelteConfigTypeChecked([
		'packages/svelte/src/**/*',
		'packages/svelte/tests/*/test.ts',
		'packages/svelte/tests/runtime-browser/test-ssr.ts',
		'packages/svelte/tests/*/samples/*/_config.js'
	])
];

I can also send PRs directly to sveltejs/svelte and sveltejs/kit repos.

[benmccann]

Some of these rules have been setup in sveltejs/svelte in sveltejs/svelte#11842

[benmccann]

Enabled some ts stuff in SvelteKit now too: sveltejs/kit#12297