Even though {Factory::InternalizeString} was declared as a template,
only two instantiations exists: uint8_t and uint16_t. Using any other
type leads to link-time errors, which is inconvenient.

This CL implements the two instantiations explicitly, and provides a
third implementation taking a {Vector<const char>}. This will be used
after the next CL, which changes {StaticCharVector} to actually return a
{Vector<const char>}.

This also avoid the cumbersome template exports.

R=leszeks@chromium.org

Bug: v8:10426
Change-Id: I3f669fae2c711ade6f5a087e59210ad457423a66
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2152837
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67207}