patch 9.0.1804: Vim9: no support for private object methods

Problem:  Vim9: no support for private object methods
Solution: Add support for private object/class methods

closes: #12920

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Yegappan Lakshmanan <yegappan@yahoo.com>

runtime/doc/todo.txt

@@ -129,6 +129,7 @@ Further Vim9 improvements, possibly after launch:
 	    or: def _Func()
     Perhaps use "private" keyword instead of "_" prefix?
   - "final" object members - can only be set in the constructor.
+  - Support export/import of classes and interfaces.
   - Cannot use class type of itself in the method (Issue #12369)
   - Cannot use an object method in a lambda  #12417
 	Define all methods before compiling them?

runtime/doc/vim9class.txt

@@ -178,6 +178,26 @@ number to the total number of lines: >
 	enddef
 
 
+Private methods ~
+If you want object methods to be accessible only from other methods of the
+same class and not used from outside the class, then you can make them
+private.  This is done by prefixing the method name with an underscore: >
+
+    class SomeClass
+	def _Foo(): number
+	  return 10
+	enddef
+	def Bar(): number
+	  return this._Foo()
+	enddef
+    endclass
+<
+Accessing a private method outside the class will result in an error (using
+the above class): >
+
+    var a = SomeClass.new()
+    a._Foo()
+<
 Simplifying the new() method ~
 
 Many constructors take values for the object members.  Thus you very often see
@@ -284,6 +304,18 @@ object members, they cannot use the "this" keyword. >
 Inside the class the function can be called by name directly, outside the
 class the class name must be prefixed: `OtherThing.ClearTotalSize()`.
 
+Just like object methods the access can be made private by using an underscore
+as the first character in the method name: >
+
+    class OtherThing
+	static def _Foo()
+	    echo "Foo"
+	enddef
+	def Bar()
+	    OtherThing._Foo()
+	enddef
+    endclass
+
 ==============================================================================
 
 4.  Using an abstract class			*Vim9-abstract-class*