Fix zero-length pattern matches at the end of strings

[akeit0]

Pattern searches skipped the position after the last character unless the pattern itself was empty. This made string.find("abc", "$") return nil instead of (4, 3) and string.match("", "a*") return nil instead of an empty string.

Include the end position in the search loops shared by find and match, and in gmatch and gsub. Patterns requiring a character still fail there, while zero-length matches now follow Lua 5.2 behavior. Update the existing gmatch("abc", "a*") expectation to include the fourth match after c.

Add 44 regression cases covering empty inputs, end anchors, optional repetitions, position captures, start bounds, iterator termination, and substitution limits. All expected results were checked against native Lua 5.2.

Validation:

• dotnet test --verbosity quiet: 556 runtime tests and 69 source-generator tests passed.
• dotnet csharpier check .: all 252 files passed using the repository's pinned formatter.
• git diff --check: passed.