[Migration: js to tsx] HeaderPrimary.js converted to HeaderPrimary.tsx

[ksvijayan06]

Fixes #3533

Changes proposed in this pull request: Convert core JS to Typescript

Reviewers should focus on: Javascript code has been re-written in Typescript language

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.

[SychO9]

Thanks for the PR!