Preliminary cleanup of the TS codebase

[mattpap]

This PR:

• updates eslint.js to match bokehjs'
• updates tsconfig.json to match bokehjs (as much as possible)
• fixes all trivial lint errors (styling related, type imports, etc.)
• fixes all errors resulting from tsconfig.json changes
  • adds missing or fixes misused override and declare modifiers
  • fixes a few stray type errors

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.64%. Comparing base (d35e55b) to head (01ebaf0).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6406      +/-   ##
==========================================
- Coverage   71.65%   71.64%   -0.01%     
==========================================
  Files         305      305              
  Lines       45730    45730              
==========================================
- Hits        32766    32765       -1     
- Misses      12964    12965       +1     

Flag	Coverage Δ
unitexamples-tests	71.64% <ø> (-0.01%)	⬇️

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[ahuang11]

Any ideas if anything in this needs changing?
https://github.com/holoviz/panel/pull/6379/files#diff-51deb854f9bbeb887380fae693dc790cf156ba1d7fbbeec58fef929efe31c9ffR1

[mattpap]

Any ideas if anything in this needs changing?

Yes, quite a few things, but we will resolve this depending on the PR merge order, because some of the required changes may not be applicable until tsconfig.json is updated.

[mattpap]

@philippjfr, maybe we could get this merged unless there are any concerns? I have some more gradual follow up work I would like to submit.