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.

[fflaten]

Adding an example of this:

function myDemo ($o) {
    $o = "second"
}

$o = "first"
$o # Go to definition here will go to first match -> reassignment on line 2

Update: Preview extension v2023.2.0 includes change from #1984 which now shows all definitions. Good enough? Matching scope is hard, so not sure if/when that will be fixed.