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

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:
```ts
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;
      `,
    },
  ],
});
```

.prettierrc.json

@@ -1,3 +1,4 @@
 {
+  "singleQuote": true,
   "trailingComma": "all"
 }

packages/eslint-plugin-internal/package.json

@@ -12,6 +12,7 @@
     "typecheck": "tsc -p tsconfig.json --noEmit"
   },
   "dependencies": {
-    "@typescript-eslint/experimental-utils": "2.25.0"
+    "@typescript-eslint/experimental-utils": "2.25.0",
+    "prettier": "*"
   }
 }

packages/eslint-plugin-internal/src/rules/index.ts

@@ -1,9 +1,11 @@
 import noTypescriptDefaultImport from './no-typescript-default-import';
 import noTypescriptEstreeImport from './no-typescript-estree-import';
+import pluginTestFormatting from './plugin-test-formatting';
 import preferASTTypesEnum from './prefer-ast-types-enum';
 
 export default {
   'no-typescript-default-import': noTypescriptDefaultImport,
   'no-typescript-estree-import': noTypescriptEstreeImport,
+  'plugin-test-formatting': pluginTestFormatting,
   'prefer-ast-types-enum': preferASTTypesEnum,
 };