rename "legacy files" -> "idiomatic files"

[jdx]

In the docs I describe this in regards to legacy files:

asdf calls these "legacy version files" so we do too. I think this is a bad name since it implies that they shouldn't be used—which is definitely not the case IMO. I prefer the term "idiomatic" version files since they're version files not specific to asdf/mise and can be used by other tools. (.nvmrc being a notable exception, which is tied to a specific tool.)

I think we should invert this and always refer to them as "idiomatic version files" as it's a better name except in this one place where we can mention that's what asdf referred to them as.

At the time of that writing this made sense because mise was new and most people were coming from asdf so the familiarity was important, however now there are as many if not more users coming to mise not via asdf so sticking to this poor naming convention is something we should avoid. As the docs describe, calling them "legacy" implies they shouldn't be used but I certainly don't think that is the case with these files.

This is more than just a docs change, the code and error messages also need to be updated.