feat: update svelte sdk to use browser 4.x

[joker23]

• removes the browser compat dependency on svelte sdk

sdk-1703
sdk-1691

This PR also adds more type definitions to the svelte sdk

---

Note

Modernizes the Svelte SDK to the v4 browser client API and surfaces an explicit initialization status.

• Replace @launchdarkly/js-client-sdk/compat initialize with @launchdarkly/js-client-sdk createClient, start(), and waitForInitialization(); update event handling to use change
• Introduce LDClient interface and richer typings (LDIdentifyResult, LDOptions, etc.); refactor internals to use LDClientBase
• Replace boolean initializing store with string initalizationState (pending → complete/failed/timeout); update LDProvider.svelte to render initializing and failed slots based on state
• Keep flag access via toFlagsProxy, and ensure watch/useFlag guard against uninitialized client
• Update tests to mock v4 client (createClient, start, waitForInitialization) and assert new init semantics

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[github-actions]

@launchdarkly/browser size report
This is the brotli compressed size of the ESM build.
Compressed size: 171236 bytes
Compressed size limit: 200000
Uncompressed size: 798447 bytes

[github-actions]

@launchdarkly/js-sdk-common size report
This is the brotli compressed size of the ESM build.
Compressed size: 25394 bytes
Compressed size limit: 26000
Uncompressed size: 124693 bytes

[github-actions]

@launchdarkly/js-client-sdk-common size report
This is the brotli compressed size of the ESM build.
Compressed size: 19354 bytes
Compressed size limit: 20000
Uncompressed size: 99752 bytes

[github-actions]

@launchdarkly/js-client-sdk size report
This is the brotli compressed size of the ESM build.
Compressed size: 23337 bytes
Compressed size limit: 25000
Uncompressed size: 81335 bytes