Make connection's getOffsetInBlock public

[BeksOmega]

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Problem

The getOffsetInBlock method is @internal, which means that external developers aren't supposed to access it.

However, it is currently used by the debug renderer in @blockly/dev-tools as well as the marker_svgs in the default renderer and zelos.

Request

The getOffsetInBlock method should be made public. The fact that it is accessed by the marker_svgs is a particularly compelling reason, because we want external developers to be able to create their own marker_svgs, and this is a significant blocker to them doing so.

Alternatives considered

N/A

Additional context

google/blockly-samples#1747

[maribethb]

All that needs to happen here is remove the @internal annotation from getOffsetInBlock method.