Add render hooks passthroughs and blockquotes (with GitHub alert support)

[bep]

Blockquote Hooks

// BlockquoteContext is the context passed to a blockquote render hook.
type BlockquoteContext interface {
	AttributesProvider
	text.Positioner
	PageProvider

	// Zero-based ordinal for all block quotes in the current document.
	Ordinal() int

	// The blockquote text.
	// If type is "alert", this will be the alert text.
	Text() hstring.RenderedString

	/// Returns the blockquote type, one of "regular" and "alert".
	// Type "alert" indicates that this is a GitHub type alert.
	Type() string

	// The GitHub alert type converted to lowercase, e.g. "note".
	// Only set if Type is "alert".
	AlertType() string
}

Allows hook templates on the form:

• render-blockquote.html
• render-blockquote-alert.html (type)
• render-blockquote-regular.html

Passthrough Hooks

// PassThroughContext is the context passed to a passthrough render hook.
type PassthroughContext interface {
	AttributesProvider
	text.Positioner
	PageProvider

	// Currently one of "inline" or "block".
	Type() string

	// The inner content of the passthrough element, excluding the delimiters.
	Inner() string

	// Zero-based ordinal for all passthrough elements in the document.
	Ordinal() int
}

Allows hook templates on the form:

• render-passthrough.html
• render-passthrough-inline.html (type)
• render-passthrough-block.html

Fixes #11927

@jmooring and others; comments? The blockquote API differs a little from your proposal. Having PlainText would be useful. but it seems that it hard to get some sensible plain text from markdown blocks, so that needs to wait; also, I have aimed for the least amount of new moving parts.

[jmooring]

I've taken the blockquotes portion for a quick test drive (looks good), and will spend more time testing this evening.

[bep]

@jmooring I just force pushed an update (which fixes the position handling)

[jmooring]

LGTM

I've tested both hooks with various scenarios, and both are working great. I am working on the docs, and will probably finish by tomorrow evening.

[bep]

Thanks, much appreciated. Ideally I would want the transform.ToMathML ready; else the passthrough hooks aren't of much obvious use. I have solved the technical bits, but I need to think about how to wire/configure it -- ideally I would want the wasm file(s) to live outside of the binary, but I may take a shortcut for the first release and mark it as "semi experimental".