This commit changes "include_js_files!" macro from "deno_core"
in a way that "dir" option doesn't cause specifiers to be rewritten
to include it.
Example:
```
include_js_files! {
dir "js",
"hello.js",
}
```
The above definition required embedders to use:
`import ... from "internal:<ext_name>/js/hello.js"`.
But with this change, the "js" directory in which the files are stored
is an implementation detail, which for embedders results in:
`import ... from "internal:<ext_name>/hello.js"`.
The directory the files are stored in, is an implementation detail and
in some cases might result in a significant size difference for the
snapshot. As an example, in "deno_node" extension, we store the
source code in "polyfills" directory; which resulted in each specifier
to look like "internal:deno_node/polyfills/<module_name>", but with
this change it's "internal:deno_node/<module_name>".
Given that "deno_node" has over 100 files, many of them having
several import specifiers to the same extension, this change removes
10 characters from each import specifier.
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>
