Add driver subproject

[NeilKleistGao]

• Implement multiple-file compilation
• Add import and export keywords
• Update ts2mls DiffTests and test files
• Import libes5.d.ts and cherry-pick from dom.d.ts
• Add UnusableLike and Unsupported types
• Add ident escaping
• Implement invocation of ts libraries in mlscript
• Allow overriding built-in types from Predef.mlsi
• Optimize NewParser.block with tail-recursion
• Fix literal typing
• Handle selections in inheritance
• Update the implementation check for declare keyword
• Enhance ts2mls:
  • Mark complex signatures as unsupported
  • Fix generation of overloading, optional parameters and var-length parameters
  • Support import/require and export parsing
  • Support export as namespace and triple slashes
  • Fix flag checking
  • Fix private members' name

[LPTK]

Thanks for extensive work!

As discusses, there are a lot of changes in here that will need to be ported to the new compiler incrementally. I believe a lot of the implementation effort can be saved, but this will still be a painstaking process.

In the meantime, I will close this with a "to revisit" label.