Add type declarations for simple-mode

[stof]

Those are based on the type definitions in wrote for codemirror5 in DefinitelyTyped, omitting the mode property of the rule that does not exist anymore, and changing meta to languageData.

The weird generic on simpleMode<K extends string>() is because typescript does not allow defining a different type for languageData when using P in string instead of a generic (that will be inferred as the the union of state names instead of the full string type)

[marijnh]

Thanks!