documentSymbols: Only report variable declarations, not variable access

[felixfbecker]

$array is only declared once, but appears for every access too. Need to

• Only use a variable assignment node, and then take the variable from there
• Only add the variable once per scope

[felixfbecker]

The same is true for global symbol search and classes that are imported with use

[felixfbecker]

Fixed in 6be53ad