Added identifiers' grammar
# Identifiers
+> **<sup>Lexer:<sup>**
+> IDENTIFIER :
+> &nbsp;&nbsp; &nbsp;&nbsp; XID_start XID_continue<sup>\*</sup>
+> &nbsp;&nbsp; | `_` XID_continue<sup>+</sup>
+
An identifier is any nonempty Unicode[^non_ascii_idents] string of the following form:
Either
