Move files from lib to src and index into src

[erikyo]

This commit relocates files from the lib directory to the src directory and places index inside src.

• Updated paths (scripts and test)
• Added typescript dependency and the required stuff to build the library

This adjustment enables publishing everything with the build process and ensures all files, including types, are contained within the same folder. Consequently, there won't be a need to publish a .d.ts file in the root directory.

While the build step becomes necessary, it guarantees that types will always remain synchronized with those from @definitely-typed. Nonetheless, someone must execute the build step.

[johnhooks]

@smhg I don't know what level of permission is required, though I am willing to do code review for @erikyo if we can get that setup.

Update: it actually looks like I can submit a review as is.