instead of embedding each file individually, embed them all into a
single embed filesystem. This is basically a noop for the current
frontend, but sets things up a little cleaner for the new frontend.
Also added an embed.FS for the source files needed to build the new
frontend. These files are not actually embedded into the binary (since
it is a blank identifier), but causes `go mod vendor` to copy them into
the vendor directory.
Updates tailscale/corp#13775
Signed-off-by: Will Norris <will@tailscale.com>
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