make all front-matter available in KS rendering

[peterbe]

Fixes #3258

In principle, this was an easy fix. Now I can make this change in the content:

diff --git a/files/en-us/web/api/headers/index.html b/files/en-us/web/api/headers/index.html
index d7e613f48..72eeaf9b3 100644
--- a/files/en-us/web/api/headers/index.html
+++ b/files/en-us/web/api/headers/index.html
@@ -9,6 +9,7 @@ tags:
   - Headers
   - Interface
   - Reference
+browser-compat: api.Headers
 ---
 <div>{{APIRef("Fetch API")}}</div>
 
@@ -118,7 +119,7 @@ myHeaders.get('Content-Type') // should return 'text/xml'
 
 <h2 id="Browser_compatibility">Browser compatibility</h2>
 
-<p>{{Compat("api.Headers")}}</p>
+<p>{{Compat}}</p>
 
 <h2 id="See_also">See also</h2>
 

and it works! It works because now any key in the front-matter becomes available in that env object you get inside the context of KS rendering.
We used to hardcode the "transfer" of slug, title, and tags. Now it just takes all and everything from the front-matter and makes it available. There is a slight exception for the front-matter tags because it could potentially be undefined but I think the KS macros expects it to always be an array. By leaving it to always fall back on an empty array, no macro code would need to do something like if (!Array.isArray(env.tags)) { or anything like that.
But yes, the env['browser-compat'] might be undefined but this is a new thing so I think it's fair for KS macros to always check for truthy'ness before using it.

I'm not entirely sure what the consequences might be related to KS rendering caching so I'm tempted to make this a draft for @escattone to ponder. But I don't usually make draft PRs if there's nothing left on my to-do list for the branch work.

[escattone]

Small but exciting change!

[wbamberg]

This is amazing :).

[Elchi3]

Wohooo! Thanks so much Peter and Ryan!

[peterbe]

Taking it for a test-drive here: mdn/content#4102

One thing I've never loved about kumascript is that it's actually perfectly fine to use {{Compat}} or {{Compat()}}. It hurts my 22-years Python muscle memory.