cmd/tsconnect: add dev-pkg command for two-sided development

Allows imports of the NPM package added by 1a093ef482
to be replaced with import("http://localhost:9090/pkg/pkg.js"), so that
changes can be made in parallel to both the module and code that uses
it (without any need for NPM publishing or even building of the package).

Updates #5415

Signed-off-by: Mihai Parparita <mihai@tailscale.com>
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