Context

Moving the stylelint-plugin from the main Sourcegraph repo to leverage logic used for codemods later.
This PR also introduces:

1. Typescript support for custom Stylelint rules.
   

   codemod/packages/stylelint-plugin/src/rules/filenames-match-regex/filenames-match-regex.ts

   Lines 14 to 18 in 3c7e6e6

   	interface RuleConfig {
   	regexp: string
   	}
   	export const filenameMatchRegexRule = createPlugin<RuleConfig>(ruleName, (isEnabled, config) => {

2. Infrastructure to test new rules.
   

   codemod/packages/stylelint-plugin/src/rules/no-restricted-imports/__tests__/no-restricted-imports.test.ts

   Lines 3 to 38 in 3c7e6e6

   	testRule({
   	ruleName,
   	config: [true, { paths: ['bootstrap*', 'reactstrap/index.css'] }],
   	codeFilename: 'packages/stylelint-plugin/test.css',
   	accept: [
   	{
   	code: '@import "reactstrap/not-banned.css"',
   	description: 'Absolute import',
   	},
   	{
   	code: '@import "./branded/src/global-styles/index.scss";',
   	description: 'Relative import',
   	},
   	{
   	code: '@import "../../branded/src/global-styles/index.scss";',
   	description: 'Deep relative import',
   	},
   	],
   	reject: [
   	{
   	code: '@import "reactstrap/index.css";',
   	description: 'Banned import exact path',
   	message: messages.rejected('reactstrap/index.css'),
   	},
   	{
   	code: '@import "bootstrap";',
   	description: 'Banned import via glob',
   	message: messages.rejected('bootstrap'),
   	},
   	{
   	code: '@import "bootstrap/scss/functions";',
   	description: 'Banned nested import via glob',
   	message: messages.rejected('bootstrap/scss/functions'),
   	},
   	],
   	})

3. Updates the stylelint package to the version used in the main Sourcegraph repo.
4. The new no-restricted-imports rule that we can use to ban imports from packages that we want to migrate from. E.g., bootstrap.
   

   codemod/packages/stylelint-plugin/src/rules/no-restricted-imports/no-restricted-imports.ts

   Line 11 in 3c7e6e6

   export const ruleName = '@sourcegraph/no-restricted-imports'

Integration plan

1. After merging this PR, the stylelint-plugin will be published to NPM.
2. Then, we can install it in the main Sourcegraph repo and drop the JS version defined there.

Test plan

1. cd packages/stylelint-plugin && yarn link
2. cd main-sourcegraph-repo && yarn link "@sourcegraph/stylelint-plugin-sourcegraph"