feature/featuretags: make QR codes modular (#18358)

QR codes are used by `tailscale up --qr` to provide an easy way to
open a web-page without transcribing a difficult URI. However, there’s
no need for this feature if the client will never be called
interactively. So this PR adds the `ts_omit_qrcodes` build tag.

Updates #18182

Signed-off-by: Simon Law <sfllaw@tailscale.com>

util/qrcodes/format.go

@@ -0,0 +1,22 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package qrcodes
+
+// Format selects the text representation used to print QR codes.
+type Format string
+
+const (
+	// FormatAuto will format QR codes to best fit the capabilities of the
+	// [io.Writer].
+	FormatAuto Format = "auto"
+
+	// FormatASCII will format QR codes with only ASCII characters.
+	FormatASCII Format = "ascii"
+
+	// FormatLarge will format QR codes with full block characters.
+	FormatLarge Format = "large"
+
+	// FormatSmall will format QR codes with full and half block characters.
+	FormatSmall Format = "small"
+)