Fix deeply nested blocks

Switches the blocks scope to using a negative lookahead end pattern
matcher instead of the while pattern.

Fixes #37.

syntaxes/rst.tmLanguage.json

@@ -234,7 +234,7 @@
 		},
 		"block": {
 			"begin": "^(\\s*)(\\.{2}\\s+\\S+::)(.*)",
-			"while": "^\\1(?=\\s)|^\\s*$",
+			"end": "^(?!\\1\\s|\\s*$)",
 			"beginCaptures": {
 				"2": {
 					"name": "keyword.control"

syntaxes/test/_nested-blocks.rst

@@ -0,0 +1,25 @@
+-- SYNTAX TEST "source.rst" "nested blocks"
+
+:param: abc
+-- <----- -keyword.control
+
+.. note:: aa
+-- <--------- keyword.control
+
+  :param: abc
+--^^^^^^^ keyword.control
+
+  .. note:: b
+--^^^^^^^^^ keyword.control
+
+    .. note:: c
+--  ^^^^^^^^^ keyword.control
+
+    .. comment c
+--  ^^^^^^^^^^^^ comment.block
+
+  .. comment b
+--^^^^^^^^^^^^ comment.block
+
+.. comment a
+-- <------------ comment.block