feat(plugins): install packages from a .zip or a folder's plugin.toml

Install Plugin from Package File now takes an `.oiplugin`, a `.zip` or a
package folder's `plugin.toml`, and goes through the same install page
(capabilities, signature, consent) as every other install.

- A zip may hold the package at its root or in its only folder, as zipping
  the folder makes; archiver metadata (`__MACOSX`, `.DS_Store`, `._*`) is
  left out. A zip without a `plugin.toml` there, such as an IntelliJ plugin,
  is refused with a message saying so. Unsafe paths, links and size limits
  are checked as for `.oiplugin` files.
- Picking a `plugin.toml` installs its folder.

Co-authored-by: florentvdry <[email protected]>

docs/plugins/themes.md

@@ -109,8 +109,9 @@ There is no build step. The package *is* the folder.
 1. Copy `templates/theme-plugin` and rename `id`, `name` and the theme files.
 2. Edit the colours. Keep `kind` truthful (`light` for light backgrounds).
 3. **Install:** *Settings › Plugins › ⚙ › Install Plugin from Disk…* (or the
-   *Install Plugin from Disk…* action) and pick the package's `plugin.toml`, or
-   a `.zip` of the folder. The manifest and the files it references are copied to
+   *Install Plugin from Disk…* action) and pick the package folder, or
+   *Install Plugin from Package File…* and pick its `plugin.toml` or a `.zip` of
+   the folder. The manifest and the files it references are copied to
    `<app-data>/plugins/<id>/`.
 4. **Apply:** in *Settings › Plugins*, select the plugin and press *Apply* next
    to the theme. *Use Default Theme* switches back.