For consistency with other `Emitter` impls, such as `JsonEmitter`,
`SilentEmitter`, `SharedEmitter`, etc.

For consistency with other `Emitter` impls.

There are three functions only used for the JSON format.

PR rust-lang#82639 changed UI tests so `-Z unstable-options` aren't passed to UI
tests by default. This completely broke `use_suggestion_json.rs`, which
uses the unstable `--error-format=pretty-json` option. The expected
output went from 400 lines of pretty JSON error messages to a single
JSON error saying "`--error-format=pretty-json` is unstable"!

This commit adds `-Z unstable-options` back and reinstates the old
expected output, with some minor changes to account for shifted spans
and slightly JSON output changes since then.

Currently for these two errors we go to the effort of switching to a
standard JSON emitter, for no obvious reason, and unlike any other
errors. This behaviour was added for `pretty-json` in rust-lang#45737, and then
`human-annotate-rs` copied it some time later when it was added.

This commit changes things to just using the requested emitter, which is
simpler and consistent with other errors.

Old output:
```
$ rustc --error-format pretty-json
{"$message_type":"diagnostic","message":"`--error-format=pretty-json` is unstable","code":null,"level":"error","spans":[],"children":[],"rendered":"error: `--error-format=pretty-json` is unstable\n\n"}

$ rustc --error-format human-annotate-rs
{"$message_type":"diagnostic","message":"`--error-format=human-annotate-rs` is unstable","code":null,"level":"error","spans":[],"children":[],"rendered":"error: `--error-format=human-annotate-rs` is unstable\n\n"}
```

New output:
```
$ rustc --error-format pretty-json
{
  "$message_type": "diagnostic",
  "message": "`--error-format=pretty-json` is unstable",
  "code": null,
  "level": "error",
  "spans": [],
  "children": [],
  "rendered": "error: `--error-format=pretty-json` is unstable\n\n"
}

$ rustc --error-format human-annotate-rs
error: `--error-format=human-annotate-rs` is unstable
```