Support local variables for completion and definition requests

[bjarosze]

Motivation

Support for completion and go to definition for local variables.

Implementation

Basic idea is that we don't have to index all local variables, because they are easy to pull off ad hoc. This way indexing time will stay the same.

Method responsible for locating local variables is Document#locate_local_variable_nodes. It returns all nodes that define local variables which are visible in given position.

Completion implementation is pretty straightforward. Just take all unique names of all local variables.
Definition jumps to closest definition node, e. g:

1| test = 1
2| test = 2
3| test = 3
4| puts test

Jumping to definition from line 4 jumps to line 3. From line 3 to line 2, etc.

Automated Tests

Added tests for both completion and definition.

Manual Tests

Write simple code like:

test = 1
puts t

You should see completion menu and then jump to definition.