Add type/const modifiers to imports

[stephenh]

I.e. imp("Foo@foo").t --> becomes a type import, imp("Foo@foo").c --> a non-type import.

Given ts-poet doesn't know the AST, it doesn't automatically know if an import is being used as a type or not, so the user needs to give us hints.

[stephenh]

This has been done via a t: prefix, i.e. imp("t:Foo@bar") turns into a type import, otherwise imports are assumed to be const imports.