cli: streamline autostart ux (#2251)

This commit adds the following changes:

- autostart enable|disable => autostart set|unset
- autostart enable now accepts a more natual schedule format: <time> <days-of-week> <location>
- autostart show now shows configured timezone
- 🎉 automatic timezone detection across mac, windows, linux 🎉

Fixes #1647

coderd/util/tz/tz_linux.go

@@ -0,0 +1,44 @@
+//go:build linux
+
+package tz
+
+import (
+	"path/filepath"
+	"strings"
+	"time"
+
+	"golang.org/x/xerrors"
+)
+
+const etcLocaltime = "/etc/localtime"
+const zoneInfoPath = "/usr/share/zoneinfo"
+
+// TimezoneIANA attempts to determine the local timezone in IANA format.
+// If the TZ environment variable is set, this is used.
+// Otherwise, /etc/localtime is used to determine the timezone.
+// Reference: https://stackoverflow.com/a/63805394
+// On Windows platforms, instead of reading /etc/localtime, powershell
+// is used instead to get the current time location in IANA format.
+// Reference: https://superuser.com/a/1584968
+func TimezoneIANA() (*time.Location, error) {
+	loc, err := locationFromEnv()
+	if err == nil {
+		return loc, nil
+	}
+	if !xerrors.Is(err, errNoEnvSet) {
+		return nil, xerrors.Errorf("lookup timezone from env: %w", err)
+	}
+
+	lp, err := filepath.EvalSymlinks(etcLocaltime)
+	if err != nil {
+		return nil, xerrors.Errorf("read location of %s: %w", etcLocaltime, err)
+	}
+
+	stripped := strings.Replace(lp, zoneInfoPath, "", -1)
+	stripped = strings.TrimPrefix(stripped, string(filepath.Separator))
+	loc, err = time.LoadLocation(stripped)
+	if err != nil {
+		return nil, xerrors.Errorf("invalid location %q guessed from %s: %w", stripped, lp, err)
+	}
+	return loc, nil
+}