Make Token.Symbol property public

[kahlkevin]

Motivation:

• Inside custom middleware, it may be desirable to understand relative order of symbols in the parsed command line (for e.g. in the case of several option types that influence a shared setting and that have a "last option specified wins" semantic).
• Best way to do understand sequence order is by inspecting the ParseResult.Tokens list. This gives Tokens, which contain embedded symbol references.
• From there, we'd like to compare the token's Symbol directly against predefined Argument and Option objects to ease identification.
• Doing this from non-internal code today requires something like if (tokenOfInterest == new Token(tokenOfInterest.Value, TokenType.Option, optionOfInterest)) { /* match */ ... }, which works because of the overridden Token.Equals method.
• It'd be much easier to say if (tokenOfInterest.Symbol == optionOfInterest) { /* match */ ... }
• No clear reason why Token.Symbol needs to be internal

[KalleOlaviNiemitalo]

This might simplify the redaction that I do in #1795.

[jonsequitur]

While this might be a useful API addition, it was added primarily for internal bookkeeping and it would need to be tested more thoroughly for a number of scenarios including cases where the token is shifted during parsing, i.e. ArgumentResult.OnlyTake.