feat: introduce env_vars alias for env-vars

[maiste]

This PR introduces an alias env_vars, for env-vars, to keep the configuration uniformity (using the underscore instead of dash).

Closes #11424

[nojb]

I don't think it is a good idea to allow both (env-vars) and (env_vars) (having more than one name for the same thing is generally bad). So (env-vars) should be renamed to (env_vars) in 3.18 (see Dune_lang.Syntax.renamed_in). That way, someone who upgrades to Dune lang 3.18 is forced to rename env-vars to env_vars.

[maiste]

I agree! But is it a good idea to introduce it as a breaking change for 3.18 and not for 4.0?

[maiste]

@nojb what do you think about allowing both but emit a warning for now with env-vars, and removing it for 4.0.0?

[rgrinberg]

I don't particularly care if we have more than one name, but I'm strongly against making breaking changes that offer no benefits to the user. So even if we introduce a new name, we need to keep supporting the old name even in 4.0. We need much more convincing reasons to break user builds.

[nojb]

I don't particularly care if we have more than one name, but I'm strongly against making breaking changes that offer no benefits to the user. So even if we introduce a new name, we need to keep supporting the old name even in 4.0. We need much more convincing reasons to break user builds.

OK!

[maiste]

OK, so let's stick to the two names, then.