[selection-api] Expose with which bidi-level caret is affiliated as well as API to set it (#57)

We need to expose information as to which bidi-level (LTR or RTL or both) the caret is associated with visually, as well as an API to specify it (e.g. add an optional argument to `setPosition` and `collapse`).

Also see issues #32 and #51.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/selection-api/issues/57