Added a new Identifier type which contains a name property

This name property contains a sanitized version of the TokenSyntax's
text property which for now only consists of trimming backticks

This acts as a convenience property to convert a TokenSyntax to an
Identifier

This isn't explicitly needed for this problem but it seems as though
the default for all types that can conform to Sendable/Hashable should

This also sets up this new type for Swift 6.0 (and the current Swift 5.10)
to pass this type around safely when needed

As well as allows Identifier to be a key in a dictionary which could be
a common scenario

When trying to create an Identifier from a non-identifier token, the
initializer should fail, returning nil

Additionally the identifier property of the TokenSyntax should also return nil