How can I implement a system which has LSP and Syntax Highlighting that combines built-in language with a custom language

[ProgrammingWithYash]

I want to write an editor which mixes html, css, js with my own custom language. Such as:

{{ with each $arr as $v at $k }}
    <div>Value: {{= $v }}</div>
{{ endwith }}

I was previously using CodeMirror 5 and it allows me to use an existing language mode like htmlmixed as base and add on top of it. Is there any way to do something similar in Monaco? Or do I need to write a complete highlighter and lsp from scratch that also incorporates html, css and js?