Add 'chws' and 'vchw' feature tags

[rubricator]

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OpenType® Specification Version 1.9 includes two feature tags, 'chws' and 'vchw', but these two tags are missing in the file fontspec-code-opentype.dtx. This PR adds them, and the descriptions for these two tags will appear in Table 5 of the file fontspec.pdf

[wspr]

Thanks for raising this as an issue. Some more work would be needed to add this features to fontspec. In order to do this, ca you provide an example of these features in use and (ideally) a free font which demonstrates them?

[rubricator]

This link provides a detailed explanation of the functions of 'chws' and 'vchw', accompanied by specific examples. Unfortunately, I haven't found any free fonts with 'chws' and 'vchw' features at the moment.