Remove immer, making the core slate package dependency-free

[nabbydude]

Description
After #5871, the only usage of immer left is in Node.fragment for creating a fragment object. This PR switches it to using the existing non-immer functions.

Issue
Mainly decreases bundle size for the slate package
Fixes #5644 by circumvention, could affect a solution posted for #4111

Context
I couldn't find any actual unit tests for fragments, but manual testing has yielded identical results. If I have the time and patience I might add some next week

also this removes tiny-warning which was entirely unused already

Checks

• The new code matches the existing patterns and styles.
• The tests pass with yarn test.
• The linter passes with yarn lint. (Fix errors with yarn fix.)
• The relevant examples still work. (Run examples with yarn start.)
• You've added a changeset if changing functionality. (Add one with yarn changeset add.)

[changeset-bot]

🦋 Changeset detected

Latest commit: 27e9fee

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
slate	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[dylans]

I guess immer is not really always. 😂

This looks fine to me, going to land another fix to hopefully clean up the failing tests.