preload: added a "hx-preload" header to hx-get and href requests

[bogdano]

Also added hx-request header to href preload requests, in case implementations rely on hx-boost + hx-target to update a #main div, for instance.

If UI elements hook into htmx:configRequest, then a preload request may trigger some client-side action before the user actually commits to executing the request. A check for an hx-preload header can be used to avoid this.

Also, if an implementation uses the pattern of hx-boosted <a href=""> tags and server-side logic to return only partials when a request contains the hx-request header, then the preload would unnecessarily load an entire page when a snippet would suffice. So, I've added an hx-request and hx-preload header to the XMLHTTPRequest as well.

Please let me know what you think about this change.

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[bogdano]

I apologize if I should have just opened an issue instead-- I didn't see any contribution guidelines until after I submitted the PR and checked the base htmx repo...

[Telroshan]

Hey,

• To remain consistent with the main lib's headers, I would use uppercase for HX prefix and the first letter of the header, as in
  https://github.com/bigskysoftware/htmx/blob/45566e126edd51bd24ff67871d662d6fc5b5934d/src/htmx.js#L3131
• Adding a test case would be nice, just to ensure the headers are properly set in all situations, and prevent future potential regressions
• No worries! There are no contribution guidelines for this repo atm indeed (at some point we'll add some), and as it's still relatively small, we can manage unsollicited PRs for now!