feat(experimental-utils): use mergable interface for settings property

[G-Rath]

This lets us do declaration merging to define settings at the plugin level e.g.

interface ESLintPluginJestSettings {
  describeBlocks: readonly string[];
  hookNames: readonly string[];
}

declare module '@typescript-eslint/experimental-utils/dist/ts-eslint/Rule' {
  export interface SharedConfigurationSettings
    extends ESLintPluginJestSettings {}
}

[typescript-eslint]

Thanks for the PR, @G-Rath!

typescript-eslint is a 100% community driven project, and we are incredibly grateful that you are contributing to that community.

The core maintainers work on this in their personal time, so please understand that it may not be possible for them to review your work immediately.

Thanks again!

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[nx-cloud]

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CI ran the following commands for commit 3bc6a8c. Click to see the status, the terminal output, and the build insights.

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	nx run-many --target=build --all --parallel
	nx run-many --target=typecheck --all --parallel

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[nx-cloud]

Nx Cloud Report

CI ran the following commands for commit fdf79a1. Click to see the status, the terminal output, and the build insights.

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Status	Command
	nx run-many --target=build --all --parallel
	nx run-many --target=typecheck --all --parallel

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[bradzacher]

isn't this unnecessary now?
i'm surprised our lint rule didn't fire on this

[bradzacher]

okay weird... none of the checks ran on your last commit

[G-Rath]

quite right, and yeah, that is weird...

I'll rebase to resolve the conflict and remove this at the same time, which'll hopefully trigger CI.

[bradzacher]

looks like you conflicted with your previous PR!
otherwise LGTM

[codecov]

Codecov Report

Merging #3556 (3bc6a8c) into master (ffbb3cf) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3556   +/-   ##
=======================================
  Coverage   92.63%   92.63%           
=======================================
  Files         325      325           
  Lines       11236    11236           
  Branches     3168     3168           
=======================================
  Hits        10409    10409           
  Misses        367      367           
  Partials      460      460           

Flag	Coverage Δ
unittest	92.63% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...ackages/experimental-utils/src/ts-eslint/Linter.ts	100.00% <ø> (ø)
...ges/experimental-utils/src/ts-eslint/RuleTester.ts	100.00% <ø> (ø)
...tal-utils/src/ast-utils/eslint-utils/predicates.ts	100.00% <0.00%> (ø)