Add PathCompletions namespace

[ghost]

…as this code is mostly independent of the rest of completions.

This is like the ImportTracker namespace in #14001. It's a component of a service that has little coupling to the rest of the service; it's 500+ lines of code with only 2 exports. Not sure if this should go in its own file too.

It's hard to see from the diff, but this just moves functions out of ts.Completions and into ts.Completions.PathCompletions. It also simplifies function isFunction to delegate to isFunctionLikeKind for most cases.