Use underscores for new system fields for global variable tiddlers

For consistency with `_canonical_uri`; unlike many system fields, the behaviour of these fields is baked into the core JS code.

core/modules/widgets/widget.js

@@ -160,11 +160,11 @@ Widget.prototype.getVariableInfo = function(name,options) {
 		if(tiddler) {
 			return processVariable({
 				value: tiddler.getFieldString("text"),
-				params: $tw.utils.parseParameterDefinition(tiddler.getFieldString("parameters"),{requireParenthesis: true}),
-				isMacroDefinition: tiddler.getFieldString("is-macro") === "yes",
-				isWidgetDefinition: tiddler.getFieldString("is-widget") === "yes",
-				isProcedureDefinition: tiddler.getFieldString("is-procedure") === "yes",
-				isFunctionDefinition: tiddler.getFieldString("is-function") === "yes"
+				params: $tw.utils.parseParameterDefinition(tiddler.getFieldString("_parameters"),{requireParenthesis: true}),
+				isMacroDefinition: tiddler.getFieldString("_is_macro") === "yes",
+				isWidgetDefinition: tiddler.getFieldString("_is_widget") === "yes",
+				isProcedureDefinition: tiddler.getFieldString("_is_procedure") === "yes",
+				isFunctionDefinition: tiddler.getFieldString("_is_function") === "yes"
 			});
 		}
 	}

core/wiki/macros/tabs.tid

@@ -1,7 +1,7 @@
 title: $:/global/tabs
 code-body: yes
-parameters: (tabsList,default,state:"$:/state/tab",class,template,buttonTemplate,retain,actions,explicitState)
-is-macro: yes
+_parameters: (tabsList,default,state:"$:/state/tab",class,template,buttonTemplate,retain,actions,explicitState)
+_is_macro: yes
 
 \?define tabs-button()
 \whitespace trim

editions/test/tiddlers/tests/data/globals/CustomWidget.tid

@@ -12,8 +12,8 @@ Sycamore!
 </$$mywidget>
 +
 title: $:/global/$$mywidget
-is-widget: yes
-parameters: (foo:"bar")
+_is_widget: yes
+_parameters: (foo:"bar")
 
 Koala! <$text text=<<foo>>/>, <$slot $name="ts-body"/>
 +

editions/test/tiddlers/tests/data/globals/Functions.tid

@@ -12,8 +12,8 @@ title: Output
 <$text text=<<this-is-one 7>>/>
 +
 title: $:/global/this-is-one
-is-function: yes
-parameters: (foo:"2")
+_is_function: yes
+_parameters: (foo:"2")
 
 [<foo>multiply[2.5]]
 +