Add support for JSON-formatted tiddler store, and make it the default (#5708)

* Add support for JSON-formatted tiddler store, and make it the default

The change to `getTiddlersAsJson()` is to allow experimentation

* Move JSON tiddlers into their own store area, and fix support for encrypted tiddlers

Also add a dummy old-style store area for backwards compatibility

The current arrangement is that JSON tiddlers will always override old-style tiddlers.

* Use the deserialiser mechanism to decode the content

* Refactor $:/core/modules/deserializers.js before we start extending it

Cleaning up the helper function names and ordering

* Drop support for the "systemArea" div

It was only used in really old v5.0.x

* Update deserializer to support JSON store format and add some tests

* Life UI restrictions on characters in fieldnames

* Add another test case

* Correct mis-merge

* Remove toLowerCase() methods applied to fieldnames

* Insert line breaks in output of getTiddlersAsJson (#5786)

Rather than have the entire store on one line, insert a line break
after each tiddler.

* Refactor #5786 for backwards compatibility

* Only read .tiddlywiki-tiddler-store blocks from script tags

Prompted by @simonbaird's comment here: https://github.com/Jermolene/TiddlyWiki5/pull/5708#discussion_r648833367

* Clean up escaping of unsafe script characters

It seems that escaping `<` is sufficient

* Add docs from @saqimtiaz

Thanks @saqimtiaz

* Docs tweaks

* Remove excess whitespace

Thanks @simonbaird

* Fix templates for lazy loading

* Remove obsolete item from release note

* Clean up whitespace

* Docs for the jsontiddler widget

* Fix whitespace

Fixes #5840

* Comments

* Fix newlines in JSON store area

* Remove obsolete docs change

Co-authored-by: Simon Baird <simon.baird@gmail.com>

core/modules/utils/utils.js

@@ -741,9 +741,8 @@ exports.isValidFieldName = function(name) {
 	if(!name || typeof name !== "string") {
 		return false;
 	}
-	name = name.toLowerCase().trim();
-	var fieldValidatorRegEx = /^[a-z0-9\-\._]+$/mg;
-	return fieldValidatorRegEx.test(name);
+	// Since v5.2.x, there are no restrictions on characters in field names
+	return name;
 };
 
 /*