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/unit.c

@@ -1,4 +1,5 @@
 #include "ctx.h"
+#include "expr/expr.h"
 #include "iterate.h"
 #include "node.h"
 
@@ -18,6 +19,13 @@ static struct token_parse_result parse_use_keyword(
 	return PARSE_RESULT(IVY_OK, 0);
 }
 
+static struct token_parse_result parse_var_keyword(
+	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_class_keyword(
 	struct ivy_parser *ctx, struct ivy_token *tok)
 {
@@ -67,6 +75,7 @@ struct ast_node_type unit_node_ops = {
 		KW_PARSER(PACKAGE, parse_package_keyword),
 		KW_PARSER(CLASS, parse_class_keyword),
 		KW_PARSER(USE, parse_use_keyword),
+		KW_PARSER(VAR, parse_var_keyword),
 	},
 	.n_symbol_parsers = {
 		SYM_PARSER(DOT, parse_dot),