chore: remove eslint, use biome instead

[filmaj]

Many lines of code, little actual change. Perhaps the only thing worth taking a look at is the biome.json file that describes the lint and formatting rules.

Changes

• gets us out of the current tough situation where eslint v9 is new but we are stuck on v8 due to the airbnb eslint config being archived and unmaintained
• removes eslint, adds biome instead. https://biomejs.dev/ is way faster (eslint executes in dozens of seconds while biome executes in under a second) and only a single dependency compared to the complicated eslint buildchain
• moves all tests from .js to .ts (commonJS to ESM) <-- this took the most work, finally at least a vast majority of tests are typesafe now. caught many errors because of this (like invalid HTTP response mocks and things of the like).

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 93.02857% with 183 lines in your changes missing coverage. Please review.

Project coverage is 91.61%. Comparing base (0374680) to head (becd125). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2006 +/- ## ========================================== + Coverage 90.26% 91.61% +1.34% ========================================== Files 34 37 +3 Lines 7666 9924 +2258 Branches 381 633 +252 ========================================== + Hits 6920 9092 +2172 - Misses 734 820 +86 Partials 12 12 ``` | [Flag](https://app.codecov.io/gh/slackapi/node-slack-sdk/pull/2006/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=slackapi) | Coverage Δ | | |---|---|---| | [cli-hooks](https://app.codecov.io/gh/slackapi/node-slack-sdk/pull/2006/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=slackapi) | `95.23% <95.00%> (+0.28%)` | :arrow_up: | | [cli-test](https://app.codecov.io/gh/slackapi/node-slack-sdk/pull/2006/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=slackapi) | `95.39% <80.10%> (-1.54%)` | :arrow_down: | | [oauth](https://app.codecov.io/gh/slackapi/node-slack-sdk/pull/2006/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=slackapi) | `77.39% <75.00%> (+0.85%)` | :arrow_up: | | [socket-mode](https://app.codecov.io/gh/slackapi/node-slack-sdk/pull/2006/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=slackapi) | `58.22% <59.37%> (-1.37%)` | :arrow_down: | | [web-api](https://app.codecov.io/gh/slackapi/node-slack-sdk/pull/2006/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=slackapi) | `96.86% <95.83%> (+0.28%)` | :arrow_up: | | [webhook](https://app.codecov.io/gh/slackapi/node-slack-sdk/pull/2006/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=slackapi) | `96.65% <ø> (+1.37%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=slackapi#carryforward-flags-in-the-pull-request-comment) to find out more.

[filmaj]

Sounds @seratch , I will follow up with another PR for that shortly, and can test it out by running it in the PR, too.