Support alternative file extensions

[dcodeIO]

This PR enables experimenting with alternative file extensions, by making the default extension overridable in asc, using the (temporary) --extension option. While this works, there are various follow-up issues obviously that need to be addressed eventually, like allowing multiple file extensions to be valid if one imports a foreign module's sources directly that are using .ts, but I guess this can wait until we know more (stdlib is extension agnostic).

Related: #1003, https://github.com/AssemblyScript/meta/issues/32

[jtenner]

I don't suppose we could do something like --allow-extensions ts,asc instead, with the default being ["ts"]?

[dcodeIO]

That's what I meant with additional work being necessary, but needs some refactoring around asc. For now this just enables experimenting with it.