fix: unambiguous repo key names, clearer config and key names, and a newer lazysecrets's files no longer stop an older one (#66)

Changes that would break configs or links once people use lazysecrets, made while nobody does yet.

- **Repo key names**: a dot or bracket inside a key is escaped (`app\.name`), so links to it resolve to that key and not to `app` → `name`. Dotenv files are flat, so their names stay as they are.
- **Config**: `updates.updateSources` is now `updates.sources`. A negative provider timeout, or `--timeout` of 0 or less, is an error instead of making every request fail.
- **Keybindings**: `lists.share` is now `lists.copyLink`, `lists.search` is `lists.goTo` and `lists.enter` is `lists.open`, matching what they do.
- **Version skew**, e.g. a lazysecrets.nvim theme file made for a newer lazysecrets:
  - an unknown theme role is skipped with a warning instead of stopping startup;
  - a state file from a newer version is read but not saved to, with a warning;
  - a link with an unknown provider suggests `lazysecrets update`.

Reviewed-on: #66
Co-authored-by: Karl-Einar Bergstrøm <karl-einarb@outlook.com>

internal/ui/app/update_test.go

@@ -122,3 +122,11 @@ func TestUpdateAsked(t *testing.T) {
 		t.Errorf("U on a dev build:\n%s", dev.screen())
 	}
 }
+
+func TestStartupWarning(t *testing.T) {
+	t.Parallel()
+	d := newDriverOpts(t, Options{Warning: "theme: skipped unknown activeBorder"})
+	if !strings.Contains(d.screen(), "theme: skipped unknown activeBorder") {
+		t.Errorf("no warning:\n%s", d.screen())
+	}
+}