Feature request to support ReStructuredText (`.rst`)

Issue #79 is getting crowded and hard to use. This issue is to make the request for **ReStructuredText (`.rst`)** support easy to follow.

Requested twice: https://github.com/Automattic/harper/issues/79#issuecomment-3057370535 and https://github.com/Automattic/harper/issues/79#issuecomment-2824069034

Read issue #79 and the two linked request comments first to understand the intended ReStructuredText scope. No files, entry points, tests, or acceptance criteria are named here, so the work is not startable until the project defines what .rst support and completion mean.

Written by the indexing model from the issue text.