Refactor YAML handling by centralizing reserved word checks and byte escaping

[moritz-gross]

both pretty_print and rules_archive have the same tables for working with YAML scalars.
Generally, I think there is much more YAML handling that can be consolidated.

I don't really know how this affects build.rs and package_rules.rs (or even what those really do in the first place).
Codex suggested to add #[path = "src/yaml_helpers.rs"], as otherwise these other crate roots don't manage to find src/yaml_helpers.rs which they depend on.

Anyway, I think the idea here makes sense, I'm mostly just not sure about how to properly wire this up in Rust / the project structure.

[github-actions]

Linux library size: 0.58 MiB (0.00%)

Revision	Release liblibmathcat.so
Base (3bbbd44)	0.58 MiB (605,312 bytes)
PR (822502a)	0.58 MiB (605,312 bytes)
Change	0 bytes (0.00%)

Built with default features, Rust 1.96.0, and Ubuntu 24.04. Workflow run.

[moritz-gross]

CI failure looks unrelated:

test Languages::zh::tw::units::without_prefix_other ... ok
test Languages::zh::tw::units::without_prefix_time ... ok
test Languages::zh::tw::units::without_prefix_powers_of_2 ... ok

failures:

---- Languages::en::alphabets::cap_cyrillic stdout ----
Error: MathCAT crash! Please report the following information: 'MathCAT crash! Please report the following information: 'MathCAT crash! Please report the following information: 'RefCell already borrowed' at src/speech.rs:2798:0
' at tests/common/mod.rs:56:0' at tests/common/mod.rs:61:0


failures:
    Languages::en::alphabets::cap_cyrillic

test result: FAILED. 5129 passed; 1 failed; 33 ignored; 0 measured; 0 filtered out; finished in 209.67s

error: test failed, to rerun pass `--test languages`
Error: Process completed with exit code 101.