Document private fields

Author: Sainan

docs/New Features/Object-Oriented Programming.md

@@ -124,18 +124,49 @@ print(human.name) -- "John"
 
 Note that if you have a local variable (or function parameter) called "parent", the `parent` expression will defer to it.
 
+## Private Fields
+
+Pluto allows you to specify if a field is 'public' or 'private'. Private fields can only be accessed by the class that defined them.
+
+```pluto
+class Human
+    public name
+    private age
+
+    function __construct(name, age)
+        self.name = name
+        self.age = age
+    end
+
+    function getAge()
+        return self.age
+    end
+end
+
+local human = new Human("John", 42)
+print(human.name) -- "John"
+print(human:getAge()) -- 42
+print(human.age) -- nil
+```
+
 ## Constructor Promotion
 
 Because a common task of `__construct` methods is to assign the value of arguments to table fields, Pluto provides a simple syntax to reduce this boilerplate:
 
 ```pluto
 class Human
-    function __construct(public name)
+    function __construct(public name, private age)
+    end
+
+    function getAge()
+        return self.age
     end
 end
 
-local human = new Human("John")
+local human = new Human("John", 42)
 print(human.name) -- "John"
+print(human:getAge()) -- 42
+print(human.age) -- nil
 ```
 
 ## Instanceof Operator

src/theme/pluto.js

@@ -13,7 +13,7 @@ Prism.languages.pluto = {
 	},
 	'boolean': /\b(?:true|false)\b/,
 	'number': /\b([\d][\d_]+|(0x[a-f\d]+)|(0b[01]+))(?:\.[a-f\d]*)?(?:p[+-]?\d+)?\b|\b\d+(?:\.\B|(?:\.\d*)?(?:e[+-]?\d+)?\b)|\B\.\d+(?:e[+-]?\d+)?\b/i,
-	'keyword': /\$|\b(?:and|as|class|pluto_class|enum|begin|break|do|else|elseif|end|for|function|goto|if|in|local|new|not|or|repeat|return|static|then|until|while|continue|switch|case|default|pluto_switch|pluto_continue|extends|export|pluto_export|pluto_use|public)\b/,
+	'keyword': /\$|\b(?:and|as|class|pluto_class|enum|begin|break|do|else|elseif|end|for|function|goto|if|in|local|new|not|or|repeat|return|static|then|until|while|continue|switch|case|default|pluto_switch|pluto_continue|extends|export|pluto_export|pluto_use|public|private)\b/,
 	'variable': /\b(?:self|parent|pluto_parent)\b/,
 	'operator': [
 		/[-+*%^&|#]|\/\/?|<[<=]?|>[>=]?|[=~]=?|\:=|\?|(?<!\w)\:|\?\.|instanceof/,