A sizable refactor

[michaelblyons]

Please test if you are so inclined! I originally set out to pull expressions apart from a new statements context, but it's gone a lot further than that, now.

New

• Grouping of syntax rules within the file
• Fully-recognized command expressions (i.e. no longer require Verb-Noun because we know it must be a command)
  This is the most likely place to find bugs.
• Classes have full body scope
• Functions have full body scope
• Desired State Configuration (DSC) contexts
• Workflow contexts
• Embed C# in here-strings if a couple common patterns are detected
• True/False/Null get extended scopes
• Simple string format placeholders
• Wildcard and path characters in unquoted strings following commands
• Regex in single-quoted strings after -match and -replace operators

Changed

• Many context names are pluralized, indicating that they will match multiple occurrences of their matches
• A new statements context restricts some contexts that were being nested everywhere.
  This is the second most likely place to find bugs.
• Types have been sorted into builtin aliases (some may be missing) and formal types
• using imports can now only come at the top of the file, as the docs specify.
• Variables can now be named according to the insane official support.
• Strings can be quoted with curly quotes if you're a masochist.

Maybe

• Better wildcard scoping
• More complex string format placeholders
• Context-aware flow controls
• Extend the default RegExp syntax (once each for single and double quoted strings?) to tune the regex flavor and add interpolations to double quoted strings

[michaelblyons]

@matteocoder Do you want to give this a try? Let me know if there are big regressions.