Add folds

Unfortunately it seems like this is not really testable with the
'tree-sitter' tooling.

If I want to check that this is working correctly, I should explore how
to make use of, e.g., neovim to automate testing.

queries/folds.scm

@@ -0,0 +1,18 @@
+[
+  (array_expression)
+  (record_expression)
+  (sequence_expression)
+  (if_expression)
+  (while_expression)
+  (for_expression)
+  (let_expression)
+  (function_declaration)
+  (primitive_declaration)
+] @fold
+
+[
+ (comment)
+ (string_literal)
+] @ignore
+
+; vim: sw=2 foldmethod=marker