JSDouble was really "double *excluding integral values*".
This was confusing, with JSDouble sometimes used assuming it was all doubles.
Rename JSDouble to JSNumNotInt to avoid future errors like #44818
Change-Id: I96bf33d5531b94aaf33b582e584702f47ed50395
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/183201
Commit-Queue: Stephen Adams <sra@google.com>
Reviewed-by: Sigmund Cherem <sigmund@google.com>