[w3c/permissions] Update clipboard permissions as per Clipboard Specification. (#209)

The [Clipboard API Spec](https://www.w3.org/TR/clipboard-apis/#clipboard-permissions) has had separate `clipboard-read` and `clipboard-write` permissions for a while now, so let's update this spec to mention this. This was mentioned in https://github.com/w3c/permissions/issues/175 as well.

This is my first specification change attempt, so please let me know if there's anything I'm missing. bikeshed spec had no errors when generating this change though.


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<a href="https://pr-preview.s3.amazonaws.com/dway123/permissions/pull/209.html" title="Last updated on Jun 2, 2020, 11:13 PM UTC (0d78d2f)">Preview</a> | <a href="https://pr-preview.s3.amazonaws.com/w3c/permissions/209/9675ebf...dway123:0d78d2f.html" title="Last updated on Jun 2, 2020, 11:13 PM UTC (0d78d2f)">Diff</a>
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/permissions/pull/209

-- Commit Summary --

  * Update clipboard permissions as per Clipboard Specification

-- File Changes --

    M index.bs (21)

-- Patch Links --

https://github.com/w3c/permissions/pull/209.patch
https://github.com/w3c/permissions/pull/209.diff

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