Explicit return type annotation for TypeScript functions

[ckcherry23]

What feature(s) would you like to see in RepoSense

Specify the return type of relevant functions in the frontend codebase. This is important because it can help catch errors early in the development process.

Is the feature request related to a problem?

Currently, many functions on the frontend do not have a return type even after the files were migrated to TypeScript. If we don't specify the return types for functions, TypeScript will try to infer it based on the function's implementation. However, this inference can sometimes be incorrect, leading to unexpected behavior or errors.

By specifying the return type explicitly, we can help TypeScript catch any type errors before the code even runs. Additionally, specifying return types can make the code more readable and self-documenting, as it makes it clear what type of value the function will return.

If possible, describe the solution

For official TypeScript docs on using return types, one can refer to https://www.typescriptlang.org/docs/handbook/2/everyday-types.html#return-type-annotations.

Additional context

Discussed in #1969

[suxiaocha123]

May I try this issue？

[ckcherry23]

While #2125 adds type annotations in the .vue files, the functions in the raw .ts files still lack return type annotations. Let's add these too in a separate PR so that we can close this issue.