refactor!: Replace Offset with opacityFrom and opacityTo in ColorEffect

[ufrshubham]

Description

ColorEffect's API was a bit confusing because it used Offset for getting values of opacity start and end from user. This PR changes that to use optional opacityFrom and opacityTo double parameters. It also adds validate checks on these value to make sure that are between 0 and 1.

Checklist

• I have followed the Contributor Guide when preparing my PR.
• I have updated/added tests for ALL new/updated/fixed functionality.
• I have updated/added relevant documentation in docs and added dartdoc comments with ///.
• I have updated/added relevant examples in examples or docs.

Breaking Change?

• Yes, this PR is a breaking change.
• No, this PR is not a breaking change.

Migration instructions

To specify start and end opacities for ColorEffect use the optional named parameters opacityFrom and opacityTo. So offset.dx should be set as opacityFrom and offset.dy should be set as opacityTo.

Related Issues

NA

[spydon]

This is on the #1938 list, but maybe it is worth doing this directly, because it is indeed really confusing.

[ufrshubham]

This is on the #1938 list, but maybe it is worth doing this directly, because it is indeed really confusing.

Didn't know this was already in that list 😅

[spydon]

Lgtm!