MDEV-34228: Support underscores in numeric literals (SQL:2023 T662)

[abhishek593]

The main contributions of this PR are:

• Added function get_numeric_token(), which is a wrapper around get_token() and it checks if there's an underscore in the current token.
• Added helper function strip_underscores(), which just erases underscores from the current token.
• Updated sql_lex.cc states (MY_LEX_NUMBER_IDENT, MY_LEX_INT_OR_REAL, MY_LEX_REAL) to permit single underscores between digits in decimal, hexadecimal, and binary literals.
• New Test Suite: Added mysql-test/main/numeric_underscores.test covering all possible cases.

New validation rules:

• Only a single underscore is allowed between digits.
• Consecutive underscores are prohibited (e.g., 1__000 is an error).
• An underscore cannot appear at the end of a numeric literal.
• An underscore cannot appear adjacent to a decimal point . or exponent marker e/E (e.g., 1_.0 and 1_e10 are invalid).
• Underscore is allowed immediately after the base prefix (e.g., 0x_FF, 0b_10).