tpl/transform: Capture and display warnings when KaTeX strict mode is warn

[jmooring]

Currently, if the KaTex strict mode is warn, we throw an error regardless of error code:

execute of template failed at <transform.ToMath>: error calling ToMath: not a function

Instead, we should capture and display the warnings as described in the KaTeX documentation. There are some related comments in the code:

hugo/internal/warpc/warpc.go

Lines 455 to 459 in e25db38

	// The console.log in the Javy/quickjs WebAssembly module will write to stderr.
	// In non-error situations, write that to the provided infof logger.
	if errBuff.Len() > 0 {
	opts.Infof("%s", errBuff.String())
	}

This isn't a pressing issue in that #13736 changed the default strict mode from "warn" to "error", and we have not documented "warn" mode.

---

Notes:

• The "not a function" message comes from here (i.e., e.message is "not a function"):
  

  hugo/internal/warpc/js/common.js

  Line 55 in f471936

  header.err = e.message;

• This is an educated guess, but I think if the user sets strict to warn, we have to pass in a JS function (instead of warn) that somehow captures an error (without throwing one) instead of calling console.warn. See https://katex.org/docs/error. If my guess is correct, this is a bit ugly and probably not worth the effort.
• I've looked at this for a few hours and have no idea how to approach it.