feat: E edits the config file

E opens your config like e opens a file: in the nvim lazysecrets runs
in, else in $VISUAL or $EDITOR, else in the system's default app. A
missing config is made first, with a link to its docs. Closing the
editor says to restart lazysecrets to use the changes.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/keys.md

@@ -59,6 +59,7 @@ match case exactly.
 | `x` | [custom commands](custom-commands.md) |
 | `L` | sign in again (shown when a login has expired) |
 | `U` | a newer lazysecrets: its changelog, and update in place (restart to use it); or look for one |
+| `E` | edit your [config](config.md), like `e` opens a file (made if it isn't there yet; restart to use it) |
 
 A revealed value is shown as it reads best: JSON indented, and PEM
 certificates as their subject, issuer, dates, names and thumbprint, with