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.
This PR refactors all internal js files (except core) to be written as
ES modules.
`__bootstrap`has been mostly replaced with static imports in form in
`internal:[path to file from repo root]`.
To specify if files are ESM, an `esm` method has been added to
`Extension`, similar to the `js` method.
A new ModuleLoader called `InternalModuleLoader` has been added to
enable the loading of internal specifiers, which is used in all
situations except when a snapshot is only loaded, and not a new one is
created from it.
---------
Co-authored-by: Bartek Iwańczuk <biwanczuk@gmail.com>
