Fixed source-map passed to istanbul-lib-instrument not being a plain object

[marekdedic]

Hi, I stumbled upon this issue while trying to instrument code previously compiled with svelte. The resulting source-map was an instance of SourceMap, not a plain object. This led me to babel/babel#11614 and eventually to istanbuljs/istanbuljs#576 where it was stated that istanbul-lib-instrument expects the source map to be a plain object - something which this plugin does not make sure of. This PR fixes that and makes the source-map always be a plain object.