Fix lookaround regexes in String.lastIndexOf and replaceLast

[kwy404]

Root cause

findLast in StringNodes finds the last regex match, then repositions the matcher on it with m.region(r.start(), r.end()).lookingAt(). A region uses opaque bounds by default, so lookahead and lookbehind cannot see the text outside the old match and the second match attempt fails. findLast then returns false even though a match exists:

"abab".lastIndexOf(Regex("a(?=b)"))      // error: does not contain a match, expected 2
"xab".replaceLast(Regex("(?<=x)ab"), "-") // "xab", expected "x-"

This affects the regex overloads of lastIndexOf, lastIndexOfOrNull, replaceLast and replaceLastMapped.

Fix

Reposition with m.find(r.start()) instead. It resets the matcher and searches the whole input from the start of the last match, so lookaround sees the full string and the same match is found again.

Test

Added a lookahead case to lastIndexOf() and a lookbehind case to replaceLast() in the api/string language snippet test. Before the fix, LanguageSnippetTests fails on api/string.pkl (lastIndexOf throws and replaceLast returns the string unchanged). After the fix it passes, and ./gradlew :pkl-core:spotlessCheck is clean.