Rust: Handle string literals with line breaks

[paldepind]

We currently do not handle string literals with line breaks. This is because . doesn't match newlines.

This PR simplifies the implementation to not use a regexp, but instead just checks the first character which relies on the fact that only string literals can start with " and only raw string literals can start with r.

[Copilot]

Pull Request Overview

This PR fixes a bug where string literals with line breaks were not being properly recognized. The issue occurred because the previous regex pattern using . didn't match newline characters.

Key Changes:

• Simplified the StringLiteralExpr implementation to check only the first character instead of using a regex
• Added test cases for multi-line string literals (both normal and raw strings)
• Updated expected test outputs to reflect the new string literal entries

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
rust/ql/test/extractor-tests/literal/literal.rs	Added test cases for multi-line normal and raw string literals
rust/ql/test/extractor-tests/literal/literal.expected	Updated expected output with new string literal entries and adjusted line numbers for subsequent tests
rust/ql/lib/codeql/rust/elements/internal/LiteralExprImpl.qll	Simplified string literal detection from regex to first-character check, fixing newline handling

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