refactor: Convert to Typescript (#388)

[nbbeeken]

Just for a sanity check this is the commit message that this merge should have:

refactor: Convert to Typescript (#388)

This is the first stage of converting the entire codebase over to
TypeScript. The goals of this step were to preserve history as
well as functional output of the compilation step. Here is a
summary of the commits involved:

 - move all files to `src`, preserving history
   this also maintains a copy of the original js source in the `lib`
   folder (without history) so we can compare the transpiled output
   for potential issues
 - add type information to all types
 - move entry point `bson.ts` to `src` folder for ease of future testing
 - run tests against transpiled module code
 - fix all linting issues with ts conversion
 - fix running manual tests with ts codebase
 - ensure we run the ts pipeline before publish
 - bump typescript version, fix last outstanding issue
 - `prepublishOnly` => `prepare`

NODE-2716, NODE-2718, NODE-2719, NODE-2721