config_writer: emit pretty multi-line sections, drop json5 crate

Previously when append_kvp created a new section or added a key, it
stuffed the "\n    " separator into the new kvp's wsc.0 (the whitespace
between its own key and colon) instead of the prior kvp's wsc.3 (the
whitespace after the prior trailing comma). Result looked like:

    lsp_servers: [...],
    learn

        : {generate_alternates
            : true,},}

The writer also didn't set any interior whitespace on the new section's
JSONObjectContext, so everything crammed onto one line — `{key: val,}`
compact, not `{\n    key: val,\n}` multi-line.

Rewrote the appender as append_kvp_pretty(object, key, value,
inner_indent, outer_indent):
- separator between kvps goes in the prior kvp's wsc.3, or if we're the
  first kvp in a fresh object, in the object's own wsc.0 (after its
  opening `{`)
- new kvp's wsc.3 carries `,\n<outer_indent>` so the parent's closing
  `}` lands correctly indented
- interior indent vs outer indent are both explicit, so we don't have
  to rewrite this logic every time we add another nesting level

New tests: new_section_exact_multiline_layout asserts byte-exact
output shape; new_section_and_key_format_cleanly verifies no key wraps
to the next line. Prior tests just substring-matched and happily passed
on the broken output — that's why this shipped in the first place.

Also: dropped the json5 crate dependency. json-five's serde feature
(default) provides the same from_str / to_string API. One fewer
dependency, and the two were doing the same job.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

Cargo.lock

@@ -493,7 +493,6 @@ dependencies = [
  "hyper",
  "hyper-util",
  "json-five",
- "json5",
  "libc",
  "log",
  "memchr",