add extension to populate path variables with request parameters

[jyrimatti]

This PR adds an extension to use request parameters to populate path variables. Used parameters are removed so they won't be sent in the query string or body anymore. Unrecognised variables are just ignored leaving the placeholder as it is.

For example, this:

<div hx-ext="path-params">
    <a hx-get="/items/{itemId}" hx-vals='{"itemId": "42", "foo": "bar"}'>test</div>
</div>

would invoke the URL /items/42?foo=bar.

Motivation is mainly that it's more or less just an API design choice whether to use query parameters, path variables, or both. When the API is under your own control and a "backend for frontend" kind of an API not meant to be used in other ways, you can of course make the decision to use just query parameters. But when interacting with external or more general APIs also path variables are often needed.

It might be nicer if Htmx supported this out-of-the-box as suggested in #1202 , but this extension provides a usable alternative.

What do you think? Any suggestions for improvements? Any ideas for a better name for the extension?

[alexpetros]

I don't mind putting this on one the website, because it comes up a lot and I think it's a good demonstration how to use extension to implement some new semantics.

If you rebase it, I'll put it up as a candidate to be merged.

[ddevienne]

The motivating example above uses hard-coded values. Is it possible to get them from a <form><input> instead?
(w/o Javascript, using only HTMX). The HTML I serve is to demonstrate a JSON API (barebone Postman / Swagger UI,
but w/o the OpenAPI spec), and I'd like users to enter route params via forms. Is that possible?

Perhaps via a #name syntax, that would read the value from <input id="name"/> ?

[jyrimatti]

The motivating example above uses hard-coded values. Is it possible to get them from a <form><input> instead? (w/o Javascript, using only HTMX). The HTML I serve is to demonstrate a JSON API (barebone Postman / Swagger UI, but w/o the OpenAPI spec), and I'd like users to enter route params via forms. Is that possible?

Perhaps via a #name syntax, that would read the value from <input id="name"/> ?

Yes, of course. The example isn't a realistic use case, just an easy example. The extension uses the data provided by Htmx, wherever it gets it (form fields, hx-vals...). It justs "moves" a parameter to the path if there's an appropriate placeholder present.