Adds new analyzer: stylelint

[scalvert]

Adds a new analyzer for stylelint, which allows us to expand our linting offering to that lint library in addition to the two existing ones.

Usage:

let config: Partial<LinterOptions> = {
  config: {
    rules: {
      'font-family-no-duplicate-names': true,
    },
  },
};

let analyzer: StylelintAnalyzer = new StylelintAnalyzer(config);
let result = await analyzer.analyze(filePaths);

Subsequent PRs are going to add more constrained types around the two classifications of analyzers: lint and ast.