chore: support insert true DTCG shadow, prep TODOs for next major

[jorenbroekema]

This prepares sd-transforms for some updates in Style Dictionary, while keeping backwards compatibility with v5.0.0.

Also added TODOs to clarify what changes to make for when sd-transforms does a breaking change to be on latest SD v5.x.x or even v6.

For now, we don't need a changeset/release for sd-transforms since I'm not removing the backwards compatibility we have in Style Dictionary until the next major release, see: https://github.com/style-dictionary/style-dictionary/pull/1643/changes#diff-a910c29ae52c0bba61129dd0dc3a9a3fee3293e35c3ecb177ec257e65119832cR1704 for more information about that.
The main goal is just to do the grunt of the work beforehand so it's easy to migrate this later :)

[changeset-bot]

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Latest commit: 7fb2939

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