GH-284: Add TypeScript definitions and tests

[jfahrenkrug]

Ported TS definitions from DefinitelyTyped, including tests.

Once a version with these definitions is released, we will need
to remove the definitions from DefinitelyTyped:
https://github.com/DefinitelyTyped/DefinitelyTyped#removing-a-package

[josephfrazier]

I'm no TS expert, but I'd be glad to include definitions in the package! Are you sure they're up to date? I'm just asking because I can't easily verify it myself, and it looks like DefinitelyTyped only has types up to version 3.0.0: https://github.com/DefinitelyTyped/DefinitelyTyped/blob/f2db20ef615bd88a605a9b8750ec8ba170e2415a/types/xregexp/index.d.ts

[jfahrenkrug]

@josephfrazier Good catch! I didn't do a simple port though. I used https://github.com/Microsoft/dts-gen to create a baseline, then manually checked the JS methods to make sure the types are correct (dts-gen uses any a lot). Please let me know if you want to have any improvements to the code!

[josephfrazier]

Cool, thanks for the explanation, dts-gen and dtslint sound pretty neat, at a glance!

[slevithan]

@jfahrenkrug This may be a lot to ask, but any ideas why dtslint v3.0.0+ (from 2 years ago, released a few days after this pull request) is failing for XRegExp? New versions of dtslint fail with error "TypeError: Cannot read property 'NameOnly' of undefined", but there are no references to "NameOnly" anywhere in the XRegExp codebase and I haven't been able to find information about dtslint v3 breaking changes.

[jfahrenkrug]

@slevithan Hey Steven, I have no idea. I'd be happy to take a look, but I'm not sure if I'll find time this week.