Replaces selection_base() and selection_extent() with selection() and
SetSelection(int, int) with SetSelection(range).
This also adds the following convenience methods to TextRange:
* reversed()
* Contains(size_t position)
* Contains(const TextRange& range)
as well as operator== for use in unit tests. When Flutter migrates to
C++20, we can replace that method with a default declaration.
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