feat: lazysecrets list prints everything lazysecrets can find, for fzf and editor pickers (#70)

`lazysecrets list` prints every source, vault and secret lazysecrets can find, one per line, for fzf, editor pickers and scripts: what Go to searches, without the TUI. What's cached prints at once; what's missing or older than `--max-age` loads in the background and streams in. It stops when the reader goes (a pick in fzf), saving what it loaded.

- Text: kind, mark (✗/⚠, ★, ◷ and the provider icon), name, status (why it couldn't be read, or expired / expires in 12d), note, account and link, tab-separated; kind first and account and link last, so new fields go between. Coloured from the theme; `--color never`, `--icons=false`, `--stars=false`, `--recent`, `--notes=false`, `--expiry=false`.
- `--format json`: an object per line, with starred, recent, lastUsed and uses, expires, enabled, note, reference and url too.
- `--kind` lists only some kinds, and doesn't load lists only other kinds need; `--cached` loads nothing.
- `lazysecrets open --action go|copy|show` skips the "what to do" menu and ends in the list, like Go to; `open` and `get` take `--account`, the account list printed, rather than asking which.
- The cache file moved to `internal/cachefile`: saves merge with the file under a lock, so list and the TUI don't undo each other.
- `ui.recentIcon` marks items used lately with ◷ in the TUI's lists too; Recent now means the last 90 days.
- --demo has stars, notes, recent use, and expired, expiring and disabled secrets.
- docs/integration.md documents get, list and an fzf picker; docs/links.md is docs/sharing.md.

Reviewed-on: #70
Co-authored-by: Karl-Einar Bergstrøm <karl-einarb@outlook.com>

internal/expiry/expiry.go

@@ -0,0 +1,41 @@
+// Package expiry says whether a secret, key or certificate has expired
+// or will soon, and how long it has left, the same way everywhere: the
+// TUI's lists and status pane, and lazysecrets list.
+package expiry
+
+import (
+	"fmt"
+	"time"
+)
+
+type State int
+
+const (
+	None    State = iota // no expiry, or not within the warning period
+	Soon                 // within the warning period
+	Expired              // already
+)
+
+// Of tells whether something expiring at expires (zero: never) has
+// expired by now or will within warn, and how long it has left.
+func Of(expires, now time.Time, warn time.Duration) (State, time.Duration) {
+	if expires.IsZero() {
+		return None, 0
+	}
+	left := expires.Sub(now)
+	switch {
+	case left <= 0:
+		return Expired, 0
+	case left <= warn:
+		return Soon, left
+	}
+	return None, left
+}
+
+// Left is a short "12d", or "5h" on the last day.
+func Left(d time.Duration) string {
+	if d < 24*time.Hour {
+		return fmt.Sprintf("%dh", max(1, int(d.Hours())))
+	}
+	return fmt.Sprintf("%dd", int(d.Hours()/24))
+}