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/ast/block.c

@@ -101,6 +101,13 @@ static struct token_parse_result parse_symbol(
 	return PARSE_RESULT(IVY_ERR_BAD_SYNTAX, 0);
 }
 
+static struct token_parse_result parse_var(
+	struct ivy_parser *ctx, struct ivy_token *tok)
+{
+	parser_push_state(ctx, IVY_AST_VAR, 0);
+	return PARSE_RESULT(IVY_OK, 0);
+}
+
 static struct token_parse_result parse_expr_begin(
 	struct ivy_parser *ctx, struct ivy_token *tok)
 {
@@ -150,6 +157,7 @@ struct ast_node_type block_node_ops = {
 	.n_state_size = sizeof(struct block_parser_state),
 	.n_node_size = sizeof(struct ivy_ast_block_node),
 	.n_keyword_parsers = {
+		KW_PARSER(VAR, parse_var),
 		KW_PARSER(END, parse_end),
 		KW_PARSER(ELSE, parse_else),
 		KW_PARSER_FALLBACK(parse_keyword),