Account for function and variable scope in language service features

[kaylaerdmann]

For the following intellisense features the entire file (or multiple files if dot sourced) is/are being searched through for variable and parameters:

• Get definition
• Get references
• Get occurrences

We need fix this to use the correct scope when searching on variables and parameters.

[daviwil]

This requires similar logic to #28 so potentially both could be developed at the same time.