oxfmt: call `prettier` for embedded languages

[Boshen]

• Add option to CLI https://prettier.io/docs/options#embedded-language-formatting

[kbrilla]

Will it also be called for html files, or other not supported formats? Would be a great thing to call lint/format from one place.

[Boshen]

Remaining work is https://prettier.io/docs/options#embedded-language-formatting for alpha @leaysgur