lang: ast: implement parsing of inline and standalone if-else statements

lang/ast/expr/expr.c

@@ -34,4 +34,10 @@ struct ast_node_type expr_node_ops = {
 		SYM_PARSER(SEMICOLON, arith_parse_semicolon),
 		SYM_PARSER(DOT, arith_parse_dot),
 	},
+	.n_keyword_parsers = {
+		KW_PARSER(IF, stmt_parse_if),
+		KW_PARSER(THEN, stmt_parse_then),
+		KW_PARSER(ELSE, stmt_parse_else),
+		KW_PARSER(END, stmt_parse_end),
+	}
 };