feat(plugins): file marketplace plugins under categories

`plugin.toml` takes `categories` (up to 3 of languages, themes, icon-themes,
linters, vcs, ai, frameworks, tools, other). A plugin that declares none is
filed under what its contributions imply (themes, icon themes, languages),
else under other; categories an IDE doesn't know are ignored, and `check`
warns about them. Index entries carry the categories, and Settings › Plugins
› Available filters by them and shows them in a plugin's details. The
official plugins declare theirs.

plugins/official/string-tools/plugin.toml

@@ -5,6 +5,7 @@ description = "Case conversion (camelCase, snake_case, kebab-case…), encoding
 authors = ["OpenIDEA contributors"]
 license = "Apache-2.0"
 change-notes = "0.1.0 — first release: 26 commands in four groups."
+categories = ["tools"]
 engines.openidea = "^1.0"
 wasm = "dist/plugin.wasm"
 # The commands are declared below, so Find Action and the keymap list them