First pass at making a new SSE interface.

SUPPORTS:
- New syntax `<div hx-sse="/url EventName" hx-swap="outerHTML" hx-target="somewhere">`

DOES NOT SUPPORT:
- triggers. Original syntax has been removed.
- unnamed events (can't make onmessage work)
- connection pooling (required before use)
- extra parameters on hx-swap, like: swap, settle, scroll, show (this should *probably* be implemented before use)

This may match the spec: https://html.spec.whatwg.org/multipage/server-sent-events.html#server-sent-events-intro
```
The default event type is "message".
```

Adding default “message” eventType if none is specified.  This matches the specifications Tom provided from the WHATWG.

Following through on part 1 of #155 (comment)

- "works" with limited testing.
- Adding/Removing EventSources to/from the pool works correctly.
- Events coming through to page correctly.
- Much testing, cleanup, and code style needed.
- Haven't yet tested multiple SSEs on a single page.

This is a first draft of a better kind of connection pooling.  Since the number of connections is unlikely to grow very large, it seems easier and more efficient to just put them all into one array, and scan through it whenever we need.

This enables a new function `auditEventListeners` which verifies that the (probably < 4) remote connections are still supporting active DOM nodes, and saves us the extra work of walking an entire sub-tree hunting for them.

I think this same structure could save us even more work if applied to WebSockets, too.  But, that effort should wait until we've confirmed that we like this for SSE.

- rewriting connection sharing to use a single array instead of a more complicated map of urls/connections.

This is much more comprehensive than my original `handleContent` function.

Still more work required to verify that this is handling all of the features we need, and then backport it into WebSocket handlers and AJAX handlers.

Sorry these are all manual cases.  If someone has suggestions on how to make these automated, I'm happy to improve them.