refactor: add tel: as safe URL protocol

[DannyJJK]

templ: v0.2.648

Currently trying to use this:
templ.URL("tel:" + phoneNumber)

results in:
about:invalid#TemplFailedSanitizationURL

I think this is because it only allows http, https and mailto, but I think tel should be added to this list. It's like mailto but for telephone numbers, so it will open up a phone application.

[a-h]

templ follows the lead of Hugo here: https://gohugo.io/functions/safe/url/ which only allows those schemes. There was a similar issue in Hugo's repo that got closed. gohugoio/hugo#5721

However, Wordpress has a much richer set: https://developer.wordpress.org/reference/functions/wp_allowed_protocols/

While Google's SafeHTML only blocks javascript URLs: https://github.com/google/safehtml/blob/be23134998433fcf0135dda53593fc8f8bf4df7c/url.go#L123

I originally went with the most restrictive of the set to be safe, but after this further research, I'm happy to add tel, ftp and ftps.

I'd take a PR for that if you want to contribute.

The code is here:

templ/runtime.go

Lines 470 to 478 in dbbb53b

	func URL(s string) SafeURL {
	if i := strings.IndexRune(s, ':'); i >= 0 && !strings.ContainsRune(s[:i], '/') {
	protocol := s[:i]
	if !strings.EqualFold(protocol, "http") && !strings.EqualFold(protocol, "https") && !strings.EqualFold(protocol, "mailto") {
	return FailedSanitizationURL
	}
	}
	return SafeURL(s)
	}