Optimize string.gsub for patterns without special characters

[gemtool]

string.gsub runs the pattern matcher at every position, even when the pattern has no special characters and can only match its own text. This adds a fast path for that case (non-anchored, non-empty patterns without special characters; ) is excluded as it is still an error in a pattern):

• occurrences are found directly (memchr for single characters, first/last character checks before memcmp otherwise), and the text between matches is copied at once
• replacement strings without % escapes are appended directly; other replacements go through add_value as before

The change is behind DFFlag::LuauOptimizeStringGsub and the flag-off branch is the original loop verbatim. Output is unchanged: a differential fuzz of 40k random calls (including error cases, replacement limits and string/table/function replacements) matches the original implementation exactly.

bench/micro_tests/test_string_gsub.lua	before	after	speedup
remove spaces	97.1 ms	23.2 ms	4.2x
replace newlines	69.5 ms	6.9 ms	10.1x
replace word	87.0 ms	12.6 ms	6.9x
no match	86.3 ms	9.1 ms	9.4x
function replacement	98.8 ms	24.3 ms	4.1x
short string	147.0 ms	72.6 ms	2.0x
every character matches	126.5 ms	56.7 ms	2.2x
separator prefix repeats	102.5 ms	10.6 ms	9.7x

Patterns with special characters take the original path and are unaffected. Added conformance tests for patterns without special characters.