[BUGFIX] Allow colon in TypoScript key

The TypoScript parser does not properly parse the := operator.
It searches for : only and hence wrongly breaks a TypoScript key apart.

Fix this by detecting the full operator sequence ":=", do not rely
on ":" only.

Resolves: #59965
Releases: 6.3, 6.2
Change-Id: I6401c87b45432d4d74c4092edeb333d4d9d5914e
Reviewed-on: https://review.typo3.org/31389
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn