Add build time math rendering

While very useful on its own (and combined with the passthrough render hooks), this also serves as a proof of concept of using WASI (WebAssembly System Interface) modules in Hugo.

This will be marked _experimental_ in the documentation. Not because it will be removed or changed in a dramatic way, but we need to think a little more how to best set up/configure similar services, define where these WASM files gets stored, maybe we can allow user provided WASM files plugins via Hugo Modules mounts etc.

See these issues for more context:

* https://github.com/gohugoio/hugo/issues/12736
* https://github.com/gohugoio/hugo/issues/12737

See #11927

deps/deps.go

@@ -23,6 +23,7 @@ import (
 	"github.com/gohugoio/hugo/helpers"
 	"github.com/gohugoio/hugo/hugofs"
 	"github.com/gohugoio/hugo/identity"
+	"github.com/gohugoio/hugo/internal/warpc"
 	"github.com/gohugoio/hugo/media"
 	"github.com/gohugoio/hugo/resources/page"
 	"github.com/gohugoio/hugo/resources/postpub"
@@ -93,6 +94,10 @@ type Deps struct {
 	// This is common/global for all sites.
 	BuildState *BuildState
 
+	// Holds RPC dispatchers for Katex etc.
+	// TODO(bep) rethink this re. a plugin setup, but this will have to do for now.
+	WasmDispatchers *warpc.Dispatchers
+
 	*globalErrHandler
 }
 
@@ -343,6 +348,9 @@ func (d *Deps) Close() error {
 	if d.MemCache != nil {
 		d.MemCache.Stop()
 	}
+	if d.WasmDispatchers != nil {
+		d.WasmDispatchers.Close()
+	}
 	return d.BuildClosers.Close()
 }