submit: use terminal links

[boneskull]

The output when gh stack submit creates a new PR looks like this:

=== Phase 3: PRs ===
✓ Created PR #1234 for boneskull/derp (https://github.com/boneskull/derp/pull/1234)

This should instead use the Hyperlink function:

gh-stack/internal/style/style.go

Lines 208 to 217 in f157268

	func (s *Style) Hyperlink(text, url string) string {
	if !s.enabled || !s.isTTY || url == "" {
	if url == "" {
	return text
	}
	return fmt.Sprintf("%s (%s)", text, url)
	}
	// OSC 8 hyperlink: ESC]8;;URLESC\TEXTESC]8;;ESC\
	return fmt.Sprintf("\x1b]8;;%s\x1b\\%s\x1b]8;;\x1b\\", url, text)
	}

gh stack submit uses this function when emitting a notice about updating a PR; see:

gh-stack/cmd/submit.go

Line 474 in f157268

fmt.Printf("Updating %s for %s (base: %s)... ", s.Hyperlink(fmt.Sprintf("PR #%d", d.prNum), ghClient.PRURL(d.prNum)), s.Branch(b.Name), s.Branch(parent))