feat: allow choosing editor in configuration file

[Metbcy]

Closes #944.

Adds an editor key to glow.yml so users can pick the editor used by glow config and the in-TUI e key without having to override $EDITOR globally. The use case from the issue is wanting micro for markdown while keeping vim as the system default.

Behavior

# editor used to open files (defaults to $EDITOR, or "nano" if unset).
# Accepts an editor name or a command with flags, e.g. "code --wait".
editor: ""

• editor: "micro" → glow uses micro, ignoring $EDITOR.
• editor: "code --wait" → multi-token commands work (the x/editor package already splits on whitespace).
• editor: "" or unset → existing $EDITOR → nano fallback chain is untouched.

Implementation note

When the key is set, the value is applied to glow's own process environment in validateOptions before editor.Cmd runs. This keeps the diff tiny (no plumbing through ui.Config, no fork of the upstream charmbracelet/x/editor package) and only mutates glow's own process, so the user's shell $EDITOR is not affected.

Verified locally

• editor: "/bin/echo override" + a stub EDITOR → fake editor invoked with the config file path.
• editor: "" + EDITOR=/bin/echo → $EDITOR is used.
• go build ./..., go vet ./..., go test ./... all pass.