Move TS modules into separate files

[benc-uk]

Made printer and validator their own module scopes, by placing them in separate files

As per this comment microsoft/TypeScript#16785 (comment) I believe this is the correct way to do this

This resolves the "Invalid module name in augmentation" error I was seeing in my project #7

[codecov]

Codecov Report

Merging #8 into master will not change coverage.
The diff coverage is n/a.

@@ Coverage Diff @@ ## master #8 +/- ## ======================================= Coverage 87.21% 87.21% ======================================= Files 5 5 Lines 391 391 ======================================= Hits 341 341 Misses 50 50

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[prantlf]

I went for a little different approach - a single index.d.ts to be match it with types in package.json too. But thank you for the idea!