Parser doesn't recognize non-breaking spaces

[cmoesel]

The parser's definition for whitespace does not include non-breaking spaces. As a result, if FSH contains a non-breaking space (possibly from cut-and-paste from a web page), the parser will fail parsing. We should add non-breaking space to the lexer rule for WS.

See: https://chat.fhir.org/#narrow/stream/215610-shorthand/topic/use.20identifier