[Read Aloud] Add fieldtrial testing config for word highlighting.

Adding this as a separate case instead of rolled into the
three features we're testing on 127 because word highlighting
is independent of voice switching and language downloads and is
128+.

Word highlighting is currently enabled by default, but adding
this config will enable us to disable word highlighting on 128
if needed.

Bug: none
Change-Id: I89b538aa53b69ceb5a11f36c01b9b14cb7d21dbc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5685039
Reviewed-by: Kristi Saney <kristislee@google.com>
Commit-Queue: Lauren Winston <lwinston@google.com>
Cr-Commit-Position: refs/heads/main@{#1325136}