Support different name for the XML root element

Currently the root element for the various XMLs (descriptor, attributes, substitutions) is called `linker` - which is confusing. The tool is called `ILLinker` and the verb we use to describe the process is "trimming". We're trying to avoid "linking" everywhere to not create confusion with platform linker.

We can't remove support for the `linker` element, too much backward compat, but we can easily teach the tool to accept a different name as well, and change our docs to refer to the new name.

Ideas:
`trim`, `trimming`, `trimmer`

Suggestions welcome.