This commit changes definition of "ExtensionFileSource", by changing
"code" field to being "ExtensionFileSourceCode" enum. Currently the enum
has only a single variant "IncludedInBinary". It is done in preparation
to allow embedders to decide if they want to include the source code in the
binary when snapshotting (in most cases they shouldn't do that).
In the follow up commit we'll add more variants to
"ExtensionFileSourceCode".
"include_js_files_dir!" macro was removed in favor "include_js_files!"
macro which can now accept "dir" option.
a1cd2a5915