[selection-api] setPosition/collapse should taken an optional argument for whether the caret is before or after a line break (#51)

Like the issue #32, we need a mechanism by which to specify whether the caret appears before or after a link break.  Alternatively, we could make the attribute we're adding in the issue #32 writable but having to call `setPosition` / `collapse` and set a property separately in order to place a caret isn't great.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/selection-api/issues/51