Add EventSource type definitions to Fable.Import.Browser

[carbon-twelve]

Hi there,
I found Fable.Import.Browser not including type definitions of EventSource.
It seems Fable.Import.Browser is the best module to place them considring that EventSource API is provided by each browser.
I'd be happy to submit a PR, but I'm not sure when I can get down to this.

[MangelMaxime]

Yes I think sending a PR to add it to Fable.Import.Browser is a good idea.

[alfonsogarciacaro]

Probably a good way is to take the type declarations, use ts2fable online to transform them and manual tweak if necessary.