examples : add C grammar (#2357)

grammars/c.gbnf

@@ -0,0 +1,42 @@
+root ::= (declaration)*
+
+declaration ::= dataType identifier "(" parameter? ")" "{" statement* "}"
+
+dataType  ::= "int" ws | "float" ws | "char" ws
+identifier ::= [a-zA-Z_] [a-zA-Z_0-9]*
+
+parameter ::= dataType identifier
+
+statement ::=
+    ( dataType identifier ws "=" ws expression ";" ) |
+    ( identifier ws "=" ws expression ";" ) |
+    ( identifier ws "(" argList? ")" ";" ) |
+    ( "return" ws expression ";" ) |
+    ( "while" "(" condition ")" "{" statement* "}" ) |
+    ( "for" "(" forInit ";" ws condition ";" ws forUpdate ")" "{" statement* "}" ) |
+    ( "if" "(" condition ")" "{" statement* "}" ("else" "{" statement* "}")? ) |
+    ( singleLineComment ) |
+    ( multiLineComment )
+
+forInit ::= dataType identifier ws "=" ws expression | identifier ws "=" ws expression
+forUpdate ::= identifier ws "=" ws expression
+
+condition ::= expression relationOperator expression
+relationOperator ::= ("<=" | "<" | "==" | "!=" | ">=" | ">")
+
+expression ::= term (("+" | "-") term)*
+term ::= factor(("*" | "/") factor)*
+
+factor ::= identifier | number | unaryTerm | funcCall | parenExpression
+unaryTerm ::= "-" factor
+funcCall ::= identifier "(" argList? ")"
+parenExpression ::= "(" ws expression ws ")"
+
+argList ::= expression ("," ws expression)*
+
+number ::= [0-9]+
+
+singleLineComment ::= "//" [^\n]* "\n"
+multiLineComment ::= "/*" ( [^*] | ("*" [^/]) )* "*/"
+
+ws ::= ([ \t\n]+)