Add multiline comments to PEG format

- Add multiline comment to PEG's format grammar, at COMMENT pattern's definition
- Add test for single line comments
- Add test for multiline one-line comments
- Add test for multiline n-line comments
- Add test for nested:
--: Single line in multiline
--: (Multiline in) multiline
--: Invalid grammar pattern definition inside multiline

derive/tests/grammar.pest

@@ -58,3 +58,23 @@ SYMBOL = { "shadows builtin" }
 
 WHITESPACE = _{ " " }
 COMMENT = _{ "$"+ }
+
+// Line comment
+
+/* 1-line multiline comment */
+
+/*
+	N-line multiline comment
+*/
+
+/*
+	// Line comment inside multiline
+
+	/*
+		(Multiline inside) multiline
+	*/
+
+	Invalid segment of grammar below (repeated rule)
+
+	WHITESPACE = _{ "hi" }
+*/

meta/src/grammar.pest

@@ -90,4 +90,5 @@ range_operator = { ".." }
 
 newline    = _{ "\n" | "\r\n" }
 WHITESPACE = _{ " " | "\t" | newline }
-COMMENT    = _{ "//" ~ (!newline ~ ANY)* }
+block_comment = _{ "/*" ~ (block_comment | !"*/" ~ ANY)* ~ "*/" }
+COMMENT    = _{ block_comment | ("//" ~ (!newline ~ ANY)*) }