[powershell mode] Improve

corrected angle bracket matching in operators (fixes block comments)
corrected digit matching regex
corrected spelling of 'identifier' in grammar dictionary
notCharacterOrDash explicitly includes lowercase characters
improved variable matching
splatted variables now match
bare '$' and '@' are errors, not variables
moved single-quoted string processing into tokenSingleQuoteString
incomplete strings are errors now
empty strings are no longer errors
added support for here-string interpolation with nesting support
added highlighting for splatted vars
removed arbitrary stream advancement prior to default error in tokenBase
enabled folding braces