client/web: extract web client from cli package

move the tailscale web client out of the cmd/tailscale/cli package, into
a new client/web package.  The remaining cli/web.go file is still
responsible for parsing CLI flags and such, and then calls into
client/web. This will allow the web client to be hooked into from other
contexts (for example, from a tsnet server), and provide a dedicated
space to add more functionality to this client.

Updates tailscale/corp#13775

Signed-off-by: Will Norris <will@tailscale.com>
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f