Add 'environment: Edit env file' command palette entry

[Atanusaha143]

Demo

Summary

Adds a new command palette command, environment: Edit env file, that lets you edit the loaded .env file(s) without leaving Posting:

• Suspends the app and opens all loaded environment files in the configured external editor ($EDITOR / $POSTING_EDITOR / the editor config setting) — the same suspend-and-edit pattern already used by the Scripts tab's Ctrl+E binding.
• On returning from the editor, variables are reloaded immediately (and env_changed_signal is published so autocomplete etc. refresh), so it works even when watch_env_files is disabled.
• The command only appears in the palette when environment files are actually loaded.
• Shows a friendly warning if no editor is configured or no env files are loaded.

Motivation

Changing a variable mid-session (e.g. swapping base_url, refreshing a token) currently requires switching to another terminal or suspending manually with Ctrl+Z. This makes it a single palette command, complementing the existing environment: Load env file command.

Testing

• Verified manually on macOS: palette command appears when env files are loaded, opens the editor (tested with nano), and reloads values on return — subsequent requests use the updated variables.
• Warning paths verified: no $EDITOR set, and no env files loaded.

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