feat(eslint-plugin-internal): add plugin-test-formatting rule

[bradzacher]

The strings that are used for eslint plugins will not be checked for formatting.
This can lead to diff noise as one contributor adjusts formatting, uses different quotes, etc.

This rule just enforces the following:

• all code samples are formatted with prettier
• all single line tests do not use backticks
• all multiline tests have:
  • no code on the first line
  • no code on the last line
  • the closing backtick indentation === property indentation
  • one of the following indentations:
    • no indentation at all
    • indentation of 1 + object indent

examples of enforced style:

ruleTester.run('foo', rule, {
  valid: [
    'const a = 1;',
    `
      const a = 1;
    `,
    `
const a = 1;
    `,
    {
      code: 'const a = 1;',
    },
    {
      code: `
        const a = 1;
      `,
    },
    {
      code: `
const a = 1;
      `,
    },
  ],
  invalid: [
    {
      code: 'const a = 1;',
    },
    {
      code: `
        const a = 1;
      `,
    },
    {
      code: `
const a = 1;
      `,
    },
  ],
});

[typescript-eslint]

Thanks for the PR, @bradzacher!

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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[codecov]

Codecov Report

Merging #1821 into master will decrease coverage by 0.24%.
The diff coverage is 81.75%.

@@            Coverage Diff             @@
##           master    #1821      +/-   ##
==========================================
- Coverage   94.96%   94.71%   -0.25%     
==========================================
  Files         159      160       +1     
  Lines        7114     7251     +137     
  Branches     2035     2074      +39     
==========================================
+ Hits         6756     6868     +112     
- Misses        155      165      +10     
- Partials      203      218      +15     

Flag	Coverage Δ
#unittest	94.71% <81.75%> (-0.25%)	⬇️

Impacted Files	Coverage Δ
...lugin-internal/src/rules/plugin-test-formatting.ts	81.75% <81.75%> (ø)