[Elnaril]

Add a new plugin that verifies, formats and minifies JSON documents using the Python standard library json module.
This PR aims to add support for one of the code formatters requested in #139.

What it does

Three actions are added to the Tools menu:

Action	Accelerator	Behaviour
Verify JSON	Ctrl+Alt+V	Reports whether the active document is valid JSON
Format JSON	Ctrl+Alt+F	Pretty-prints with 4-space indentation
Minify JSON	Ctrl+Alt+M	Removes insignificant whitespace

The result is reported in the status bar.
Invalid JSON leaves the document unchanged and reports the error (with line/column).
Verify works on read-only documents; Format/Minify are disabled for them.
The status bar message is cleared when the active document changes or is modified, so it cannot go stale.

Notes for reviewers

• The plugin is opt-in: it is not marked Builtin, so it must be enabled in Preferences → Plugins.
• It uses only the Python standard library (json) — no new dependency.
• It is wired into the build (plugins/meson.build), its translatable strings are listed in po/POTFILES.in, and it is documented in the user manual.
• The .po files are intentionally left untouched.

Screenshots

Thank you !