TokenSyntax.rawText to get raw token text as SyntaxText

Declare `SyntaxText` as a SPI. and expose `tokenView.rawText`.
`SyntaxText` is a byte sequence conforms to `RandomAccessCollection`
and has basic query methods e.g. '==', 'hasPrefix(_:)', 'contains(_:)'.
It is useful for performance critical logic because it's faster than
`TokenSytnax.text` which requies intantiating `TokenKind` often with
`Swift.String`.

Author: rintaro

Sources/SwiftSyntax/TokenSyntax.swift

@@ -75,6 +75,11 @@ public struct TokenSyntax: SyntaxProtocol, SyntaxHashable {
     return tokenKind.text
   }
 
+  @_spi(RawSyntax)
+  public var rawText: SyntaxText {
+    return tokenView.rawText
+  }
+
   /// The leading trivia (spaces, newlines, etc.) associated with this token.
   public var leadingTrivia: Trivia {
     get {