Improve completion AST node accuracy

[msujew]

Closes #1266

Improves the accuracy with which the completion provider picks the AST node:

1. Fixes a very minor issue in the findLeafNodeAtOffset method, which sometimes gave back a CST node that appeared after the specified offset.
2. Only pick one AST node, ideally at the position before the current completion position.

Also reduces the amount of false-positive completion proposals by implementing a performNextTokenCompletion method. In cases where we definitely first want to complete the current ID/word/keyword/etc, the completion provider should not yield more completion contexts. This method controls that behavior by checking whether we have a word at the current offset position.

[sailingKieler]

I'm basically fine with theses changes, and I would happy if my remarks would somehow make it into the code 😉