Add server sent events (#5279)

* Create server-sent-events.js

* Create sse-change-listener.js

* Implement server sent events

* Convert to ES5 and wrap in function

* Use the host string from tiddlyweb

* Improve comments in sse-server.js

* Can't use object reference as key

* Add retry timeout

* Fix a bug

* bug fix

* Fix formatting

* Fix ES5 compat

* capitalize comments

* more fixes

* Refactor tiddlywek/sse-server.js

* Extract helper functions for handling wikis and connections.
* Replace JSDoc comments.
* Fix formatting according to TW core.
* Simplify the logic for adding and removing connections.

* Fix formatting of tiddlyweb/sse-client.js

Fix formatting according to TW core.

* Fix formatting of server-sent-events.js

Fix formatting and comments following TW core guidelines.

* Extract a debounce function in sse-client.js

* Avoid using startsWith in server-sent-events.js

startsWith is part of ES2015, while TiddlyWiki uses the 5.1 dialect.

* New sse-enabled WebServer parameter

* If not set to "yes", disabled SSE request handling.
* Add documentation for the parameter in core/language/en-GB/Help/listen.tid
* Add new tiddler editions/tw5.com/tiddlers/webserver/WebServer Parameter_ sse-enabled.tid

* Disable polling for changes if SSE is enabled

* Add sse_enabled to /status JSON response
* Store syncer polling status in $:/config/SyncDisablePolling
* Handled disabling polling in core/modules/syncer.js

* Simply boolean logic in syncer.js

* Delete trailing whitespaces in syncer.js

Co-authored-by: Arlen22 <arlenbee@gmail.com>

core/modules/server/routes/get-status.js

@@ -22,6 +22,7 @@ exports.handler = function(request,response,state) {
 		username: state.authenticatedUsername || state.server.get("anon-username") || "",
 		anonymous: !state.authenticatedUsername,
 		read_only: !state.server.isAuthorized("writers",state.authenticatedUsername),
+		sse_enabled: state.server.get("sse-enabled") === "yes",
 		space: {
 			recipe: "default"
 		},