cmd/tailscale/cli, util/qrcodes: format QR codes on Linux consoles (#18182)

Raw Linux consoles support UTF-8, but we cannot assume that all UTF-8
characters are available. The default Fixed and Terminus fonts don’t
contain half-block characters (`▀` and `▄`), but do contain the
full-block character (`█`).

Sometimes, Linux doesn’t have a framebuffer, so it falls back to VGA.
When this happens, the full-block character could be anywhere in
extended ASCII block, because we don’t know which code page is active.

This PR introduces `--qr-format=auto` which tries to heuristically
detect when Tailscale is printing to a raw Linux console, whether
UTF-8 is enabled, and which block characters have been mapped in the
console font.

If Unicode characters are unavailable, the new `--qr-format=ascii`
formatter uses `#` characters instead of full-block characters.

Fixes #12935

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

cmd/tailscale/depaware.txt

@@ -47,7 +47,7 @@ tailscale.com/cmd/tailscale dependencies: (generated by github.com/tailscale/dep
         github.com/peterbourgon/ff/v3                                from github.com/peterbourgon/ff/v3/ffcli+
         github.com/peterbourgon/ff/v3/ffcli                          from tailscale.com/cmd/tailscale/cli+
         github.com/peterbourgon/ff/v3/internal                       from github.com/peterbourgon/ff/v3
-        github.com/skip2/go-qrcode                                   from tailscale.com/cmd/tailscale/cli
+        github.com/skip2/go-qrcode                                   from tailscale.com/util/qrcodes
         github.com/skip2/go-qrcode/bitset                            from github.com/skip2/go-qrcode+
         github.com/skip2/go-qrcode/reedsolomon                       from github.com/skip2/go-qrcode
    W 💣 github.com/tailscale/go-winio                                from tailscale.com/safesocket
@@ -189,6 +189,7 @@ tailscale.com/cmd/tailscale dependencies: (generated by github.com/tailscale/dep
         tailscale.com/util/must                                      from tailscale.com/clientupdate/distsign+
         tailscale.com/util/nocasemaps                                from tailscale.com/types/ipproto
         tailscale.com/util/prompt                                    from tailscale.com/cmd/tailscale/cli
+     💣 tailscale.com/util/qrcodes                                   from tailscale.com/cmd/tailscale/cli
         tailscale.com/util/quarantine                                from tailscale.com/cmd/tailscale/cli
         tailscale.com/util/rands                                     from tailscale.com/tsweb
         tailscale.com/util/set                                       from tailscale.com/ipn+