[tinymce] Add key index types for html.Schema return vales (#35249)
[tinymce] Add key index types for html.Schema return vales (#35249)

For the various get*Elements() functions in TinyMCE's Schema class, it returns
a mapping from a string to an empty object. This commit updates the return type
for all of these functions to reflect that it is an object with string keys.
georgemarshall/DefinitelyTyped,mcliment/DefinitelyTyped,georgemarshall/DefinitelyTyped,georgemarshall/DefinitelyTyped,markogresak/DefinitelyTyped,dsebastien/DefinitelyTyped,dsebastien/DefinitelyTyped,georgemarshall/DefinitelyTyped