bug(developer): add ldml compiler warning for `<layers formId="us"><layer id="base">` pattern

[mcdurdin]

TODO-LDML: we should probably add a compiler warning + unit test for <layers formId="us"><layer id="base">, because this pattern does not make sense: when using non-touch forms, the <layer> element should use modifiers attribute, and correspondingly, modifiers attribute should not be used when formId is touch.

From #12402