lang: add var keyword for variable declarations

the var keyword allows greater control over what scope a
particular variable exists in. it clarifies whether a new
variable is being defined or an existing variable is being
assigned to. it will also facilitate the implementation of
global variables.

lang/lex.c

@@ -46,6 +46,7 @@ static struct lex_token_def keywords[] = {
 	LEX_TOKEN_DEF(IVY_KW_GET, "get"),
 	LEX_TOKEN_DEF(IVY_KW_SET, "set"),
 	LEX_TOKEN_DEF(IVY_KW_END, "end"),
+	LEX_TOKEN_DEF(IVY_KW_VAR, "var"),
 };
 static const size_t nr_keywords = sizeof keywords / sizeof keywords[0];
 
@@ -1154,6 +1155,7 @@ const char *ivy_keyword_to_string(enum ivy_keyword keyword)
 		ENUM_STR(IVY_KW_GET);
 		ENUM_STR(IVY_KW_SET);
 		ENUM_STR(IVY_KW_END);
+		ENUM_STR(IVY_KW_VAR);
 	default:
 		return "";
 	}