Convert some common classes/utils to TS

[dsevillamartin]

Changes proposed in this pull request:

• Convert Application, Session, utils/extractText
• Update states/AlertManagerState and utils/RequestError

Reviewers should focus on:

• Properly typing extract is close to impossible because we override it with a different type. We use it and just pass xhr.responseText, but Mithril's extract option takes xhr and options.

  PhpStorm screenshots

  
  

• Do we want the Application types to be exported at the top or a different file?

Screenshot

Confirmed

• Frontend changes: tested on a local Flarum installation.

[davwheat]

Rebased onto master. I've dropped Application's conversion in favour of more recent progress on #3006. I didn't realise you did it in here, but the new PR is more likely to be up-to-date with new changes, and has support for new ItemList typings from #3005.

[askvortsov1]

Before review, this should be rebased, and tested against the current TypeScript environment to ensure that it doesn't introduce any new bugs.

[dsevillamartin]

@askvortsov1 I rebased this PR a week ago, that's why I re-requested the reviews.

[askvortsov1]

@askvortsov1 I rebased this PR a week ago, that's why I re-requested the reviews.

Right, but the Typecheck and type coverage GH actions aren't showing up, so those must have been merged after you rebased.

[davwheat]

Looks good to me!

Not sure if these suggestions might be better as Babel compiles them to a smaller bundle size. Up to you.

[dsevillamartin]

@davwheat I'd rather keep it as is, I feel like String() is clearer.