[TASK] Improve StringUtility

This patch adds the methods beginsWith() and endsWith().
Furthermore it deprecates the (Core-wise unused) isLastPartOfString()
method, because we slightly change the implementation of it, which is
now part of endsWith().

Resolves: #59132
Resolves: #45499
Releases: 6.3
Change-Id: Ida21a85a3cb724618a490f7eab753978a1bfaa70
Reviewed-on: https://review.typo3.org/30611
Reviewed-by: Susanne Moog
Tested-by: Susanne Moog
Reviewed-by: Markus Klein
Tested-by: Markus Klein