Avoid quadratic block comment indent allocation

[SantanDon]

Fixes#38.

Summary

OSS-Fuzz issue 63220 reports a fuzz_parse timeout on a malformed JSON5 input. The timeout is driven by block-comment indentation handling: add_block_comment() constructs an indent-sized String for every block comment, even when the comment is single-line and the string is never used.

The published OSS-Fuzz reproducer contains 141,675 block-comment starts, many at large column offsets. Across that input, the existing code requests roughly 7.67 billion bytes worth of transient indent strings while parsing an 817,615-byte file.

This change checks the required leading-space prefix directly in the comment line instead of allocating a new indent string. Multiline alignment behavior is unchanged.

Reproducer / performance

Using the public reproducer from OSS-Fuzz issue 63220 on current master (25a594b7daacb4605241dc8d63e7c36411208c87):

input size: 817,615 bytes
before: 4,720,363 us (release build)
after: 289,199 us (release build)

The parser returns the same error result in both cases. This is about a 16x reduction on the minimized testcase; the OSS-Fuzz ASan job reported the original input exceeding its 60-second timeout.

Validation

• cargo test --lib: 45 passed
• integration test binary: 38 passed after compiling the existing test target with --cap-lints allow; current tests/lib.rs otherwise fails at the crate-level #![deny(missing_docs)] lint before tests run, which is already the subject of bugfix: fix missing_docs error #44
• cargo test --example formatjson5: 2 passed
• rustfmt --check src/parser.rs: passed

No API or output-format changes are intended.