fix: specify sourceFileName when generating inline sourcemaps

[JLHwung]

Q	A
Fixed Issues?	Fixes #10786
Patch: Bug Fix?	Y
Tests Added + Pass?	Nope
License	MIT

In this PR we specify script url as the sourceFileName option when doing in-browser transforms, otherwise the generated sourcemap have incorrected sources fields
i.e.

On a script /foo/script.js

var a;
var b;

babel will generate the following source map since the default sourceFileName is path.relative(filename).

{"version":3,"sources":["script.js"],"names":["a","b"],"mappings":";;AAAA,IAAIA,CAAJ;AACA,IAAIC,CAAJ","sourcesContent":["var a;\nvar b;"]}