vim-patch:9.2.1107: getregionpos() is slow when only the region bounds are needed (#41925)

Problem:  getregionpos() returns a pair of positions for every line, so
          obtaining just the outer endpoints of a region costs O(lines);
          for a 100000 line region that is about 1.4 million list
          allocations for a result of which the caller keeps two
          positions.
Solution: Add a "bounds" item to {opts} that returns a single pair with
          the start on the first line and the end on the last line
          (Volodymyr Chernetskyi).

The lines in between do not influence the outer endpoints, so with
"bounds" they are not visited at all. The two positions then come from
different lines and describe a diagonal of the region rather than its
shape; for a blockwise region they are the start of the first line and
the end of the last line, not the corners of the block.  This is exactly
what indexing the full result gives, so the awkward corners are kept as
they are, including an empty first line yielding a "col" of 0 for the
start while the end keeps its own column.

On a 100000 line buffer this takes getregionpos() from about 45 ms to
about 0.003 ms per call.

closes: vim/vim#21209

https://github.com/vim/vim/commit/2173c3e4eceedc76c4fd7d56cdbfbbd35e9be853

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