Add missing package exports and tree-shaking documentation

[Copilot]

The library was missing 21 package.json exports and lacked documentation on optimal import patterns for tree-shaking.

Changes

Package Exports

• Added 11 missing component exports (collapsible, container, contextmenu, divider, icon, image, layer, link, root, tree, virtual)
• Added 5 missing atoms subpath exports (breadcrumb/atoms, collapsible/atoms, contextmenu/atoms, layer/atoms, tree/atoms)
• Added 5 missing subcomponent exports (datagrid/td, datagrid/th, datagrid/tr, dropdown/item, portal/portals)
• Sorted all 83 exports alphabetically

Source Fixes

• Created missing index.ts files for incomplete components
• Populated empty atoms.ts file for contextmenu
• Removed duplicate list export from components barrel

Documentation

• Added comprehensive tree-shaking guide at docs/TREE_SHAKING.md
• Added tree-shaking section to README.md

Import Pattern

Consumers should use subpath imports for optimal tree-shaking:

// Recommended - only bundles what you use
import { Button } from '@svelte-atoms/core/button';
import { Card } from '@svelte-atoms/core/card';

// Avoid - may bundle more than needed
import { Button, Card } from '@svelte-atoms/core';

The package is configured with sideEffects: ["**/*.css"] to enable aggressive tree-shaking. All components are now accessible via 83 subpath exports.

Original prompt

analyze the current imports and expors and provide a refactoring plan for improving tree-shacking process

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