set `upload_sourcemaps` to true by default

[buenaflor]

Not sure if there is a historic reason for it to be opt-in but I'd argue it's easier for the user to set up and the docs will be less.

looked at the first commit and it has always been false by default, I don't think there is much to argue against it being opt-out

[buenaflor]

any concerns for this one? @denrase @kahest

imo it makes a lot of sense and couldn't find anything speaking against this change so far.

although we might need another major for this?

[kahest]

@buenaflor I don't have context and can't think of reason why this is opt-in, maybe double-check with the team e.g. @krystofwoldrich @romtsn

[buenaflor]

Me neither unfortunately, it has been opt-in since the very first commit of the plugin

[buenaflor]

Might be worth releasing another dart plugin major with source maps upload by default + debug id injection by default