Replace the janky GML parser with a good one

[adam-coster]

The current GML parser used by the VSCode extension can only handle globals. It isn't smart enough to fully parse a project, and can't do any advanced things like scope analysis.

For full Intellisense support we really need scope analysis, so we also need a new parser.

The MVP needs to include at least the functionality we currently have.

MVP Features

• Generate concrete syntax tree from GML
• Track all project global declarations
  • macros
  • enums
  • globalvars
  • script-functions
  • assets
• Track all project global references
  • macros
  • enums
  • globalvars
  • script-functions
  • built-ins
  • asset IDs
• Global function signatures
• 🆕 Track all localvar declarations
• 🆕 Track all localvar references
• 🆕 Track all enum member references
• Watch files for changes (e.g. for catching external changes)
• Force a virtual file-change (e.g. for current, unsaved edits)
• 🆕 Fault-tolerant parsing
• Re-parse a single file on change
• List all symbol locations and types in a given file (for semantic highlighting)
  • 🆕 vars
  • built-in functions
  • built-in constants
  • 🆕 enum members
• For a given file and position, list all in-scope (for autocomplete)
  • 🆕 vars
  • globals
  • 🆕 enum members
  • built-ins
• For a given file and position, get the symbol at that position if there is one (for hovers etc)
• 🆕 Create diagnostics based on parser errors
• 🆕 Create diagnostics based on unknown variables