Add hx-prompt-default to set the default value of a hx-prompt (#601)

[maekoos]

Description

Added hx-prompt-default attribute to (optionally) set the default value of a hx-prompt.
Also updated the documentation to include this new attribute.

Corresponding issue: #601

Testing

I added a manual test to confirm-and-prompt.html.

Checklist

• I have read the contribution guidelines
• I have targeted this PR against the correct branch (master for website changes, dev for
  source changes)
• This is either a bugfix, a documentation update, or a new feature that has been explicitly
  approved via an issue - not explicitly approved
• I ran the test suite locally (npm run test) and verified that it succeeded

[MichaelWest22]

May not be best to add too many additional hx- attributes to htmx where possible. Here we could keep the feature much more contained by just adding an appropriate separator that is unlikely for an existing user to have added to a existing hx-prompt question like | and make it take hx-prompt="do you like cats?|yes" . then it is just a .split() and pass in the two array values to prompt I think. Not sure if there is a better separator character to choose.

[maekoos]

Sure, but that would also require more sanitisation on the server side - most template engines already sanitize attributes, so it's nice to not have to think about it....

Would it be any better to rename it to something else? hx-prompt::default?

[MichaelWest22]

templating engines may sanitize attribute names a bit but we are just talking about a attribute string value which should only have basic sanitizing to prevent escaping out of the "". This is why it is important to pick a good sensible text character to split on. needs to be a character that won't upset engines and is unlikely to be used by existing hx-prompt user questions. most htmx attributes use colon ":" as the seperator but it is likely someone could have used this inside a prompt already. It is also common in htmx to use a word as the seperator and this could work as well. "do you like cats?default:yes" for example with split("default:") in the code as an example.

[maekoos]

Most template engines don't sanitize attribute names at all - what I meant was that I can't do

hx-prompt="How old is {{ userName }}?|{{ defaultAge }}"

Since I'd have to look for | in the userName variable in case someone intentionally tries to break stuff...

Also, unless a really weird character is chosen we'd also have to allow escaping (like \|), and so a regex or similar is probably necessary.

[maekoos]

Plus, reusing the hx-prompt attribute is not backwards compatible enough imo.