refactor: drop dead code and use the standard library where it already does the job (#76)

No behaviour change.

- Deleted, unused or one-line wrappers: panes.RuneIndexes, Detail.Refresh (SetDescribed does it), SecretValue.Len, Query.Empty, Clipboard.Native, config.Load, get's value, Model.done, providerOf (cachefile.ProviderOf), contextWithTimeout, searchCoverage, startCrawl, showKeybindings, the expiry aliases and daysLeft.
- cmp.Or instead of the two cmpOr copies; slices.Sorted(maps.Keys(…)), maps.Copy, wg.Go, utf8.RuneCountInString and new(true) instead of loops and helpers that do the same.
- A container's pane title is set once, not twice; a redundant length check in share links and a no-op assignment in linkhandler.Launch are gone.
- Tests call ansi.Strip and lastLines directly; gofmt -s on links.go.

Reviewed-on: #76
Co-authored-by: Karl-Einar Bergstrøm <karl-einarb@outlook.com>

internal/match/match.go

@@ -107,9 +107,6 @@ func parseTerm(tok string) (term, bool) {
 	return t, true
 }
 
-// Empty reports whether the query has no terms: everything matches.
-func (q Query) Empty() bool { return len(q.groups) == 0 }
-
 // Item is a row to match: Text is everything searched, and runes
 // NameStart up to NameEnd of it are the row's name (the rest is context,
 // like a path or a note). Without a name, nothing gets the name bonus.