chore: add eslint as linter

[pano9000]

Hi,

this PR adds an initial eslint configuration that we can use for linting our code base.
https://typescript-eslint.io/

It still needs some rule refinement for our current stage, because we still have lots of "any" types, which it complains about with the default rules

✖ 1030 problems (1030 errors, 0 warnings)
  60 errors and 0 warnings potentially fixable with the `--fix` option.

majority is complains about use of "any".

I'll likely work on the rules first, before marking the PR as ready :-)

closes TriliumNext/Trilium#5524
closes TriliumNext/Trilium#5026

[pano9000]

so, I've ran a quick count on the errors using
npm run dev:linter-check | grep -Po "@.+$" | sort | uniq -c | sort -hr

    363 @typescript-eslint/no-explicit-any
    244 @typescript-eslint/no-unused-vars
     50 @typescript-eslint/no-empty-object-type
     22 @ts-expect-error" instead of "@ts-ignore", as "@ts-ignore" will do nothing if the following line is error-free  @typescript-eslint/ban-ts-comment
     15 @typescript-eslint/no-wrapper-object-types
      5 @typescript-eslint/no-unused-expressions
      3 @typescript-eslint/no-require-imports
      2 @typescript-eslint/no-this-alias
      1 @typescript-eslint/no-unsafe-function-type

(edit: the data above is only showing the typescript-eslint related entries, the "regular" eslint rules are missing, just if anyones wondering, why the numbers don't add up).

The "no explicit any" error is not that bad after all – I though it would've been a higher count, after doing a first scroll through :-D
I'd maybe say let's keep the check active and keep it as reminder to get rid of as much any as possible – gradually.
(and in the cases, where it is not really possible → we can use eslint ignore comments)

@eliandoran what do you think?

[pano9000]

quick aside, regarding failing CI:
since we now depend on TypeScript 5.8.2, we will need to wait until typescript-eslint is updated to support 5.8 as well

looks like it will land soon though :-)
typescript-eslint/typescript-eslint#10903

[pano9000]

ok, it has been released -> will update and rebase the branch accordingly later this evening:
https://github.com/typescript-eslint/typescript-eslint/releases/tag/v8.26.0

[pano9000]

PR ready for a first start :-)
I'll keep this PR without actual eslint fixes – I'd do these in (one or several) separate steps :-)