Improve SymbolProvider.ts

[msftrncs]

Overhaul of SymbolProvider.ts to more thoroughly process well known POU symbols excluding symbol discovery in comments or quoted text, and with recovery of parsing on syntax errors where ever possible.

• This adds POU detection of INTERFACE, METHOD, ACTION keywords.
• This adds symbol discovery for VAR_GLOBAL and others.
• This adds symbol discovery for all VAR's even when not nested in appropriate POU's.
• This prevents symbols being inappropriately detected in commented areas or quoted strings.
• This adds symbol discovery for comma separated variable definitions.
• This corrects discovery of symbols following the attribute keywords, such as RETAIN, CONSTANT, PERSISTENT, PUBLIC, PRIVATE, etc...
• This adds symbol discovery for multiple instances of the same VAR keyword, often utilizing different attributes.
• This adds detection of CONSTANT symbols.
• This adds discovery of non STRUCT based types (simple types).
• This adds discovery of symbols in UNION structures.
• This improves, for all symbols, the navigation and breadcrumb features in VS Code, by providing the full document positions for the symbol.
• This attempts to stop discovery of symbols with common keywords, which may occur during normal editing, cutting and pasting, etc...
• This may have altered the original vscode.SymbolKind selections for some symbols.
• This may have dropped some description elements for some symbols.
• This may still need some refinement on symbol description for complex symbol definitions, ex (POINTER TO ARRAY[x..y] OF FB_ProcessSomething.
• This PR should be considered a work in progress until it has been communicated (from either side) otherwise. (and this line removed)

References issue #25.

[msftrncs]

Already fixed an issue where AT % address definitions failed the symbol discovery for the entire VAR block.

[Serhioromano]

I've tested and it works fine. Although it works very slow. Anyway, could you please resubmit this PR against develop branch? I use Git Flow, so master is the latest release.