support templating languages: `gohtml`

With the new emergence of htmx, no extension currently handles perfectly the htmx data extension on VSCode because the customData field is not extendable on those templating languages, even though we are writing a lot of HTML inside.

In a package.json, I would expect this to work:

	"contributes": {
		"html": {
			"customData": [
				"./html.htmx-data.json"
			]
		},
		"gohtml": {
			"customData": [
				"./html.htmx-data.json"
			]
		}
	}

Empirical tests on my fork of a fully working extension for html files: https://github.com/otovo/htmx-tags/compare/main...grouville:htmx-tags:main show that it does not

Start with the package.json contributes example and the linked htmx-tags comparison, then inspect how this repository represents HTML customData and templating-language support. Confirm whether gohtml can accept the same customData entry and validate the behavior against the htmx example; the issue names no test or implementation file.