[Pitch] Adding HTML coverage support

I have a WIP implementation here: swiftlang/swift-package-manager#9076

To try it, build the project and then invoke the built swift-test binary directly. On *nix system, this could look like:

❯ swift build
❯ $(swift build --show-bin-path)/swift-test --help
<...SNIP..>
COVERAGE OPTIONS:
  --show-codecov-path, --show-code-coverage-path, --show-coverage-path
                          Print the path of the exported code coverage JSON file.
  --print-codecov-path-mode, --print-code-coverage-path-mode, --print-coverage-path-mode <print-codecov-path-mode>
                          How to display the paths of the selected code coverage file formats. (default: text)
        json              - Display the output in JSON format.
        text              - Display the output as plain text.
  --enable-codecov, --enable-code-coverage, --enable-coverage/--disable-codecov, --disable-code-coverage, --disable-coverage
                          Enable code coverage. (default: --disable-codecov)
  --codecov-format, --code-coverage-format, --coverage-format <format>
                          Format of the code coverage output. Can be specified multiple times. (values: json, html; default: Produces a JSON coverage report.)
<...SNIP...>

The help text for the --coverage-format option does not appear correctly as each item should have an associated help text. This is tracked by apple/swift-argument-parser#818.

The PR currently allows specifying multiple coverage format by providing --coverage-format <format> multiple times.

What works well? What needs to be improved?

Sam