Add new inline_css, inline_js, and inline_head keys on the
AssetProvider::getAssets() function. This allows us to support
collectors that require static assets that are not actually saved to a
file.
Then, update all the asset functions in JavascriptRenderer to support
these new keys.
An initial use case for this is supporting the HtmlDumper in Symfony’s
VarDumper. HtmlDumper only provides the styles and scripts in inline
HTML form. The static assets can be customized based on some
configuration properties available on the HtmlDumper class. One can
actually view the CSS/JS as a long PHP string/heredoc embedded in the
HtmlDumper.php source code. They are only accessible via the
getDumpHeader function, which returns the CSS/JS in a combined HTML
string.