feat: remove the lib directory from source control; add types meta

[marcusrbrown]

I noticed some older source files in the lib folder. This change will remove those files from source control. I also added the types field to package.json as a hint to software that looks for type declarations (VS Code, online REPLs, etc.).

[thejustinwalsh]

Just to make sure I follow, the idea here is that lib is only needed in the final package sent to npm and is generated currently via publish.

Maybe there should also be a prepare / prepublishOnly script that calls build to ensure bad packages do not make it out to npm? Slightly outside the scope of this change and can be done in a follow up: #18.