This would be very useful for ocaml.org! (ocaml/ocaml.org#1181)

Note to self for searchability: (dialect) requires Dune 1.11, which probably doesn't require telling the users about because Dream already typically asks people to use newer Dune, and there are probably few users of such old versions anyway.

Thank you! This is a huge improvement, and I will merge this.

We need a few things from Dune to make this fully ergonomic:

1. Drop the requirement for a dummy (interface). Since we are going to recommend these stanzas to users, it's best for them to be brief and to have nothing unnecessary.
2. Support for files with multiple extensions, since we are currently recommending .eml.ml files. Long-term, if there is syntax support, we might switch to .eml files, but it might be possible for Dune to support files with multiple extensions.

@tmattio, @gpetiot, @emillon, I've opened two issues on Dune (ocaml/dune#7776, ocaml/dune#7777) about improving the ergonomics, and I think I will merge this regardless. I might tweak the command line before release, and deprecate the "old" instructions for integrating eml into projects. Very glad to learn about (dialect).

Thank you! I will update the docs to describe this in a follow-on commit.

Since Dream's examples recommend .eml.ml files, documenting this is blocked on ocaml/dune#7777 ("Support multiple extensions in (dialect) patterns") in Dune, which will probably come out in the next release. This is already usable from Dream master with single extensions, like .eml, as found in the Ocaml.org sources.