Rename htmx.ajax() context to options

[scriptogre]

The third htmx.ajax() argument contains options used to build the request context. Calling both objects context makes them difficult to distinguish.

Before:

htmx.ajax(method, url, context)
HtmxAjaxContext

After:

htmx.ajax(method, url, options)
HtmxAjaxOptions

Changes

1. Rename the runtime argument from context to options.
2. Rename HtmxAjaxContext to HtmxAjaxOptions.
3. Update docs and test names.
4. Link options to the request context shape.

JavaScript calls and runtime behavior do not change.