runtime(java): Improve the recognition of literals (#14120)

runtime(java): Improve the recognition of literals (#14120)

* Emend the Unicode and octal escape sequence patterns;
* Accept the (repeated) underscore separators in all
  numerical literals;
* Recognise hexadecimal floating-point literals.

(The space escape sequence '\s' will be introduced along
with text blocks in another PR.)

References:
https://docs.oracle.com/javase/specs/jls/se17/html/jls-3.html#jls-3.3
https://docs.oracle.com/javase/specs/jls/se17/html/jls-3.html#jls-3.10.1
https://docs.oracle.com/javase/specs/jls/se17/html/jls-3.html#jls-3.10.2
https://docs.oracle.com/javase/specs/jls/se17/html/jls-3.html#jls-3.10.7

Signed-off-by: Aliaksei Budavei <0x000c70@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>