Even though both are allowed in the style guide, it recommends to use
'using', as its syntax is more consistent with the rest of C++.
This CL turns all typedefs in src/regexp to 'using' declarations.

R=jgruber@chromium.org

Bug: v8:8834
Change-Id: I2765c3465fec7e8c42c3a84b924522f220ab5676
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1545904Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60585}