[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/support_table.md

@@ -453,7 +453,7 @@ use `\ce` instead|
 |\hookleftarrow|$\hookleftarrow$||
 |\hookrightarrow|$\hookrightarrow$||
 |\hphantom|$a\hphantom{bc}d$|`a\hphantom{bc}d`|
-|\href|$\href{https://katex.org/}{\KaTeX}$|`\href{https://katex.org/}{\KaTeX}`|
+|\href|$\href{https://katex.org/}{\KaTeX}$|`\href{https://katex.org/}{\KaTeX}` Requires `trust` [option](options.md)|
 |\hskip|$w\hskip1em i\hskip2em d$|`w\hskip1em i\hskip2em d`|
 |\hslash|$\hslash$||
 |\hspace|$s\hspace7ex k$|`s\hspace7ex k`|
@@ -1105,7 +1105,7 @@ use `\ce` instead|
 |\upsilon|$\upsilon$||
 |\upuparrows|$\upuparrows$||
 |\urcorner|$\urcorner$||
-|\url|$\footnotesize\url{https://katex.org/}$|`\url{https://katex.org/}`|
+|\url|$\footnotesize\url{https://katex.org/}$|`\url{https://katex.org/}` Requires `trust` [option](options.md)|
 |\utilde|$\utilde{AB}$|`\utilde{AB}`|
 
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