Add true and false keywords to the parser.

include/lexer.h

@@ -60,6 +60,8 @@ enum libab_lexer_token_e {
     TOKEN_OP_PREFIX,
     TOKEN_OP_POSTFIX,
     TOKEN_OP_RESERVED,
+    TOKEN_KW_TRUE,
+    TOKEN_KW_FALSE,
     TOKEN_KW_IF,
     TOKEN_KW_ELSE,
     TOKEN_KW_WHILE,

include/tree.h

@@ -19,6 +19,8 @@ enum libab_tree_variant_e {
     TREE_POSTFIX_OP,
     TREE_BLOCK,
     TREE_VOID,
+    TREE_TRUE,
+    TREE_FALSE,
     TREE_IF,
     TREE_WHILE,
     TREE_DOWHILE,