Use whisker for html

Summary:
Whisker is a replacement for mustache and is largely backward compatible. It brings several quality of life improvements like... error checking.

You can read more about the details [in this post](https://fb.workplace.com/groups/498666380550247/permalink/2051389461944590/).

This diff moves `t_mstch_html_generator` to use whisker instead of mstch.

This requires some changes to the templates to work around the bugs in mstch that have been previously worked around.

#buildall

Reviewed By: dtolnay

Differential Revision: D67569371

fbshipit-source-id: adf9968a91ea8c24ce0d3c0deafba2e2bc66d341

third-party/thrift/src/thrift/compiler/generate/t_mstch_html_generator.cc

@@ -22,6 +22,12 @@ class t_mstch_html_generator : public t_mstch_generator {
  public:
   using t_mstch_generator::t_mstch_generator;
 
+  std::optional<whisker_options> use_whisker() const override {
+    whisker_options opts;
+    opts.allowed_undefined_variables = {};
+    return opts;
+  }
+
   std::string template_prefix() const override { return "html"; }
 
   void generate_program() override {