Pointing to the exact spot of the incorrect numeric separator seems
clearer both in terms of the error itself, and the resulting code in
scanner because we don't have to keep track of the start position.

Previously, the error was:
  ➜ ./out.gn/x64.release/d8 --harmony-numeric-separator -e '0x1__1'
  unnamed:1: SyntaxError: Only one underscore is allowed as numeric separator
  0x1__1
  ^^^^
  SyntaxError: Only one underscore is allowed as numeric separator

Now, the error is:
  ➜ ./out.gn/x64.release/d8 --harmony-numeric-separator -e '0x1__1'
  unnamed:1: SyntaxError: Only one underscore is allowed as numeric separator
  0x1__1
      ^
  SyntaxError: Only one underscore is allowed as numeric separator


Bug: v8:7317
Change-Id: I7df1b39816e51a97234da6ed0fca1bf8c0223c3e
Reviewed-on: https://chromium-review.googlesource.com/962241
Commit-Queue: Sathya Gunasekaran <gsathya@chromium.org>
Reviewed-by: Mathias Bynens <mathias@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51950}