I just added HEEX to the sections:
[language_servers.emmet-language-server]
language = "HTML"
languages = ["HEEX", "HTML", "PHP", "ERB", "JavaScript", "TSX", "CSS"]
[language_servers.emmet-language-server.language_ids]
TSX = "typescriptreact"
HEEX = "heex"
HTML = "html"
PHP = "php"
JavaScript = "javascriptreact"
CSS = "css"
ERB = "eruby"
