Lightning_FlowScreen support

[JeroenSfdc]

This will support Lightning_FlowScreen as target. See issue #66

Description

• Added the necessary Lightning_FlowScreen target
• Removed datasource attributes as these are not supported today by Flow.

Motivation and Context

#66
Flow is a very relevant target to show Charts.

How Has This Been Tested?

Ceated a demo Flow.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

This can break for package upgrades, as there is a bug with updating an pre-existing targets, when a Lightning_FlowScreen is being added. GUS Work Item: W-9032710

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[JeroenSfdc]

@victorgz any chance you feel this PR could be a reasonable enhancement?