Fix GH-21027: strtotime() incorrectly parses decimal relative values

[khaledalam]

Description

Fixes #21027

strtotime("2.37685 weeks") returned incorrect result (~414 million seconds instead of ~1.4 million) because:

• "2.37" matched timeshort24 pattern (hour:minute with dot separator)
• "685 weeks" was parsed as separate relative offset

Changes

• Added relnumberdec regex pattern for decimal numbers
• Added relativedec lexer rule to match decimal relative expressions
• Added timelib_get_decimal_nr() helper to parse decimal values
• Added timelib_set_relative_decimal() to convert decimal units to seconds

[devnexen]

You need to open a PR in the original repository instead.