[breaking] trust setting to indicate whether input text is trusted (#1794)

* trust option to indicate whether input text is trusted

* Revamp into trust contexts beyond just command

* Document new trust function style

* Fix screenshot testing

* Use trust setting in \url and \href

* Check `isTrusted` in `\url` and `\href` (so now disabled by default)
* Automatically compute `protocol` from `url` in `isTrusted`, so it
  doesn't need to be passed into every context.

* Document untrusted features in support list/table

* Existing tests trust by default

* remove allowedProtocols and fix flow errors

* remove 'allowedProtocols' from documentation

* add a comment about a flow error, rename urlToProtocol to protocolFromUrl

* add tests test that use function version of trust option

* default trust to false in MathML tests

* fix test title, remove 'trust: false' from test settings since it's the default

docs/supported.md

@@ -101,6 +101,13 @@ The `{array}` environment does not yet support `\cline` or `\multicolumn`.
 
 ## HTML
 
+The following "raw HTML" features are potentially dangerous for untrusted
+inputs, so they are disabled by default, and attempting to use them produces
+the command names in red (which you can configure via the `errorColor`
+[option](options.md)).  To fully trust your LaTeX input, you need to pass
+an option of `trust: true`; you can also enable just some of the commands
+or for just some URLs via the `trust` [option](options.md).
+
 |||
 |:----------------|:-------------------|
 | $\href{https://katex.org/}{\KaTeX}$ | `\href{https://katex.org/}{\KaTeX}` |