[expressions] Add repeat function, reverse variant for strings (Request in QGIS)

[qgis-bot]

Request for documentation

From pull request qgis/QGIS#61335
Author: @nyalldawson
QGIS version: 3.44

[expressions] Add repeat function, reverse variant for strings

PR Description:

• Adds a new 'repeat' function, which repeats a string a specified number of times
• Adds string variant for 'reverse' to allow reversing strings

Commits tagged with [need-docs] or [FEATURE]