Improve ts declarations

[dragomirtitian]

Fixes #12. Also other general improvements.

[andersk]

Consider rebasing this onto #16. (They currently conflict because #16 dedents everything to remove the incorrect declare module wrapper.)

[dragomirtitian]

@andersk Sure I can do that. But do these PR have a path to being merged? #16 has been quiet for a half a year. Is @7rulnik still actively maintaining this project? I ask because we are looking at adopting this but #25 is really a show stopper since we have to reimplement fromSourceMap which is technically doable but worrying.

[7rulnik]

@dragomirtitian hi! Well, I guess we need to define «actively».

At the start of the fork, I did not expect any further contributions other than those described in the @mraleph blog post.

But as we can see there is a demand for it. For now, this project has 29kk installs a week which is pretty huge. Because of this I kinda worried about change that I don't have time to test by myself or don't have experience to judge it.

Despite all of the above, I am open to merging this and other pull requests.

[dragomirtitian]

@7rulnik That is great to hear. For sure I get the reluctance to make any big changes, and objectively there probably is no need. If it works why fix it :)

[7rulnik]

I rebased it onto #16

[7rulnik]

Thanks for PR and sorry for waiting!
Published as https://www.npmjs.com/package/source-map-js/v/1.2.1