ucm: add DefineRegex 'all' scheme for multiple pattern matches (Syntax 9)

ucm: add DefineRegex 'all' scheme for multiple pattern matches (Syntax 9)

Extend DefineRegex to support two matching schemes:
- "first" (default): matches pattern once, extracts capture groups
- "all": matches pattern multiple times, extracts all matches

The "all" scheme creates variables with naming pattern nameN for Nth
match and nameN_M for Nth match's Mth capture group, enabling
extraction of repeating patterns from strings.

Updated documentation with detailed examples and comprehensive
description of regex flags, schemes, and variable naming conventions.

Signed-off-by: Jaroslav Kysela <perex@perex.cz>