Remove one allocation when parsing quoted strings

[sebastienros]

No description provided.

[Copilot]

Pull Request Overview

This PR optimizes quoted string parsing by eliminating an unnecessary TextSpan allocation. The changes introduce a new overload of DecodeString that accepts string, offset, and length parameters directly, allowing the code to avoid creating an intermediate TextSpan object when the string contains no escape sequences.

Key changes:

• Added DecodeString(string, int, int) overload to avoid allocating TextSpan when no escapes are present
• Refactored escape sequence processing into DecodeStringInternal to share logic across overloads
• Updated StringLiteral parser to use the new optimized method signature

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
src/Parlot/Character.cs	Adds new DecodeString overload and refactors internal logic for string decoding
src/Parlot/Fluent/StringLiteral.cs	Updates to use the new DecodeString(string, int, int) method signature
test/Parlot.Tests/CharacterTests.cs	Adds tests for the new DecodeString overload with offset/length parameters
test/Parlot.Tests/CompileTests.cs	Adds test coverage for both compiled and non-compiled string literal parsing

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