[selection-api] Make some arguments optional (#30)

In both WebKit and Blink, the arguments of some functions are optional. That includes some nonsensical cases like "void addRange([Default=Undefined] optional Range range);" but also some arguments that have sensible defaults:
collapse's offset argument (0)
extend's offset argument (0)
containsNode's allowPartialContainment argument (false)

setBaseAndExtent also has all optional arguments, but that makes less sense.

Consider making at least the first three cases optional, as it does not seem worthwhile investigating the Web compat impact of making them non-optional.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/selection-api/issues/30