Rename "Undo Expand Region"

[mfulton26]

VSCode has built-in "Expand Select" and "Shrink Select" while this great extension has "Expand Region" and "Undo Expand Region". "Undo" seems to imply that it only works if you first "Do" (i.e. "Expand Region") which is not accurate as it can shrink the selected region without every using "Expand Region" previously.

I almost reported the same issue as #14 before finding it. I believe it would be more intuitive and consistent with VSCode built-ins to rename "Undo Expand Region" to "Shrink Region".