deprecate no-unreachable once implemented in TS compiler

[adidahiya]

compiler is adding reachability checks soon: microsoft/TypeScript#4788

[adidahiya]

Looks like it'll be released in TS 1.8: https://github.com/Microsoft/TypeScript/wiki/Roadmap

[glen-84]

1.8 is released now. =)

[adidahiya]

@jkillian chatted about this and there is some merit to keeping the TSLint rule around in case users don't want to turn on the compiler error (which would fail builds) but leave it as a linter warning (which may not fail builds). Thoughts?

Personally I'm on board with removing the rule in TSLint 4.0 just to avoid duplicating compiler logic. But if there are objections I could be convinced to keep it around.