chore: Migrate the content-highlight plugin to Typescript. (#1855)

* chore: Rename index.js to index.ts

* fix: Explicitly declare instance variables

* fix: Add types and visibility modifiers to methods

* fix: Remove underscores from names of private methods

* fix: Clean up some comments

* fix: Remove trailing underscores

* chore: Clean up code a bit

* chore: Format/lint

* chore: Use camel case instead of constant case to satisfy the linter