Add typescript guideline and typescript-specific eslint plugins and fix issues (#31521)

1. Add some general guidelines how to write our typescript code
2. Add `@typescript-eslint/eslint-plugin`, general typescript rules
3. Add `eslint-plugin-deprecation` to detect deprecated code
4. Fix all new lint issues that came up

docs/content/contributing/guidelines-frontend.en-us.md

@@ -79,6 +79,22 @@ We use htmx for simple interactions. You can see an example for simple interacti
 Although mixing different frameworks is discouraged,
 it should also work if the mixing is necessary and the code is well-designed and maintainable.
 
+### Typescript
+
+Gitea is in the process of migrating to type-safe Typescript. Here are some specific guidelines regarding Typescript in the codebase:
+
+#### Use type aliases instead of interfaces
+
+Prefer to use type aliases because they can represent any type and are generally more flexible to use than interfaces.
+
+#### Use separate type imports
+
+We use `verbatimModuleSyntax` so type and non-type imports from the same file must be split into two `import type` statements. This enables the typescript compiler to completely eliminate the type import statements during compilation.
+
+#### Use `@ts-expect-error` instead of `@ts-ignore`
+
+Both annotations should be avoided, but if you have to use them, use `@ts-expect-error` because it will not leave ineffective statements after the issue is fixed.
+
 ### `async` Functions
 
 Only mark a function as `async` if and only if there are `await` calls