Refactor DroppableContainers object into a custom Map instance

[clauderic]

Fixes #125

Refactored DroppableContainers type from Record<UniqueIdentifier, DroppableContainer to a custom instance that extends the Map constructor and adds a few other methods such as toArray(), getEnabled() and getNodeFor(id)

[changeset-bot]

🦋 Changeset detected

Latest commit: aa9349c

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

This PR includes changesets to release 3 packages

Name	Type
@dnd-kit/core	Major
@dnd-kit/sortable	Major
@dnd-kit/modifiers	Major

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[github-actions]

Size Change: +401 B (+1%)

Total Size: 60.7 kB

Filename	Size	Change
packages/core/dist/core.cjs.development.js	15.9 kB	+154 B (+1%)
packages/core/dist/core.cjs.production.min.js	10.1 kB	+116 B (+1%)
packages/core/dist/core.esm.js	15.7 kB	+135 B (+1%)
packages/sortable/dist/sortable.cjs.development.js	3.9 kB	-1 B (0%)
packages/sortable/dist/sortable.cjs.production.min.js	2.55 kB	+2 B (0%)
packages/sortable/dist/sortable.esm.js	3.78 kB	-5 B (0%)

ℹ️ View Unchanged

Filename	Size
packages/accessibility/dist/accessibility.cjs.development.js	640 B
packages/accessibility/dist/accessibility.cjs.production.min.js	486 B
packages/accessibility/dist/accessibility.esm.js	503 B
packages/accessibility/dist/index.js	148 B
packages/core/dist/index.js	141 B
packages/modifiers/dist/index.js	144 B
packages/modifiers/dist/modifiers.cjs.development.js	837 B
packages/modifiers/dist/modifiers.cjs.production.min.js	613 B
packages/modifiers/dist/modifiers.esm.js	760 B
packages/sortable/dist/index.js	144 B
packages/utilities/dist/index.js	144 B
packages/utilities/dist/utilities.cjs.development.js	1.64 kB
packages/utilities/dist/utilities.cjs.production.min.js	989 B
packages/utilities/dist/utilities.esm.js	1.54 kB

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