Fix Markdown docs with source-like syntax

[subotac]

Fixes #1480.

Markdown doc text was passed through Source::push_str, which treats braces as generated-code structure. A doc line beginning with } could therefore reduce the tracked indentation before the
surrounding documentation block was deindented, causing an overflow panic.

Add a literal append path that preserves indentation without interpreting source syntax, use it for Markdown documentation, and cover the reported input with regression tests.

Tests

• cargo test -p wit-bindgen-core -p wit-bindgen-markdown
• cargo clippy -p wit-bindgen-core -p wit-bindgen-markdown --all-targets -- -D warnings
• cargo fmt --all -- --check
• CLI generation using the reported WIT input