Add Rust-style string continuation escapes

[thurn]

Summary

• parse Rust-style string continuation escapes in standard strings and byte strings
• accept LF and CRLF line endings and strip the same following whitespace as Rust
• document the grammar change and add it to the changelog

Motivation

RON currently treats a backslash immediately before a newline as an unknown escape. Supporting Rust-style continuations lets multiline string literals follow the surrounding indentation without including that newline and indentation in the deserialized value.

This implements the parser and grammar update requested in #585 (comment).

Impact

Standard string and byte-string literals can now use line continuations. Raw strings, character literals, and serialization output are unchanged.

Validation

• cargo test --test escape
• cargo test --all-features --all-targets
• cargo check --no-default-features
• cargo fmt --all -- --check

• I've included my change in CHANGELOG.md

[juntyr]

Thanks @thurn for your PR! I will merge once you have addressed my two tiny nits.