Move ExternalReference to own files

This is a move-only CL (except includes cleanup).

Bug: v8:6666
Change-Id: I28422b4a21074e642e8d5c9b3a563ecafdafbda7
Reviewed-on: https://chromium-review.googlesource.com/957087Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51838}

BUILD.gn

@@ -1654,6 +1654,8 @@ v8_source_set("v8_base") {
     "src/extensions/trigger-failure-extension.h",
     "src/external-reference-table.cc",
     "src/external-reference-table.h",
+    "src/external-reference.cc",
+    "src/external-reference.h",
     "src/factory-inl.h",
     "src/factory.cc",
     "src/factory.h",