Improve variable detection

[fflaten]

Rewritten variable detection to:

• Validate variable names against character rules defined in about_Variables
• Validate scope in scoped variables
• Fix special character variable matching. Current version used greedy match that didn't stop at the correct }
• Detect variables inside arrays, subexpression, dictionary/hashtable or variable used as dict key

Other changes:

• Lots of variable detection tests added for better coverage
• Moved patterns and other constants to class-level

Fix #9

[fflaten]

@TylerLeonhardt Please review. Learning TS/JS as I go here 🙂

[fflaten]

Remaining scopes not added as supported:

• workflow: Ignored as it's niche and not available in cross-platform.
• function: Would require expression, but ignored as it's usually multi-line, so not suitable.
• Can be implemented using expression values (see Support environment variables #12)
  • Alias: Aliases defined in the current scope
  • Env: Environment variables defined in the current scope

[TylerLeonhardt]

Looks fantastic! Awesome work 👏

[TylerLeonhardt]

This has now been published in 0.0.6