feat: format INI files

A port of prettier-plugin-ini 1.3.0: a file the plugin reads prints as
the plugin prints it, with spaces around = by default. A file with a
line the plugin rejects formats in kept mode: each line keeps its
indent, and a line indented deeper than its key stays as written, so
configparser values and WPT metadata keep their meaning. git config
files are always in kept mode.

The plugin is unstable in two places, and tidy differs there: an empty
section gets one blank line after it, not one more on each pass, and a
list without values is a key with an empty value.