Fix Facet Parsing

[drasticactions]

In the documentation, I had wrote that you need to get the byte slice to create a facet. For example:

// To insert a link, we need to find the start and end of the link text.
// This is done as a "ByteSlice."
int promptStart = prompt.IndexOf("Link Goes Here!", StringComparison.InvariantCulture);
int promptEnd = promptStart + Encoding.Default.GetBytes("Link Goes Here!").Length;
var index = new FacetIndex(promptStart, promptEnd);
var link = FacetFeature.CreateLink("https://drasticactions.dev");
var facet = new Facet(index, link);

This is "kinda" right (You do need to get the bytes from UTF8, not rely on the default from .NET, and the start index may not represent the actual place in the byte array), but not only is this not totally correct, the helpers I wrote to make them were not using this. They were using Length from the matched Regex. This would break with Unicode/non-ASCII characters.

This should fix it by using spans to pattern match.

To use the new helpers, you can use Facet.Parse for a complete string, or Facet.ForMentions, Facet.ForHashtags, Facet.ForMentions for the individual facets.