ci/tests: move to biome, v4 linter fixes

[filmaj]

To be merged into #2254 - seperating out the linting changes from the rest for ease of reviewing #2254

Changes

• 🔪 eslint and nyc, 🤗 biome and c8
• "safe" biome linter fixes applied
  • including prefixing node native imports with node:; fixes Use node:-prefixed imports for Node.js core modules #1803
• some quality-of-life additions to .gitignore
• test folders were moved around a bit, mostly due to the testing expanding in size and there being multiple different kinds of tests:
  • tests now exist under the test/ directory
  • test/unit/ contains all unit tests, powered by Mocha
  • test/types contains all type tests, powered by tsd. Love was put into these tests. Expanding tsd tests is the best way to ensure we don't regress on TypeScript-developer-experience-related bits like autocomplete.
• the old App-context-types.spec.ts file was removed and instead those 'unit' tests were ported over to actual type tests (using tsd)
• a lot of type casting was removed where it would not introduce breakage. we should stay clear of type casting (e.g. as Whatever) as much as possible and lean on the type system to do its thing.
• test helpers under test/unit/helpers were expanded: now have more dummy event creators that lean into the type system rather than avoid it
• examples changes:
  • linted and ensured the examples/ work with bolt v4. also added the custom-receiver example to the list of examples TS integration testing w/ bolt
  • removed hubot-example

TODO

• update all examples

[codecov]

Codecov Report

Attention: Patch coverage is 93.95349% with 52 lines in your changes missing coverage. Please review.

Please upload report for BASE (bolt-four@6ab4e47). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
src/App.ts	92.24%	18 Missing ⚠️
src/receivers/HTTPModuleFunctions.ts	91.14%	17 Missing ⚠️
src/receivers/HTTPReceiver.ts	81.08%	14 Missing ⚠️
src/receivers/AwsLambdaReceiver.ts	95.00%	1 Missing ⚠️
src/receivers/ExpressReceiver.ts	98.70%	1 Missing ⚠️
src/receivers/SocketModeReceiver.ts	97.36%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             bolt-four    #2259   +/-   ##
============================================
  Coverage             ?   90.79%           
============================================
  Files                ?       20           
  Lines                ?     5576           
  Branches             ?      602           
============================================
  Hits                 ?     5063           
  Misses               ?      505           
  Partials             ?        8           

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