feat: Add "when" property to Fallback component

[webJose]

Fixes #44.

[Copilot]

Pull Request Overview

This PR adds a new "when" property to the Fallback component that allows custom control over when fallback content should be displayed. This addresses issue #44 by providing override capability for the default fallback logic which only shows content when no routes match.

Key changes:

• Added WhenPredicate type for custom fallback conditions
• Modified Fallback component to support conditional rendering via the when prop
• Created comprehensive test suite covering all routing universes

Reviewed Changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/lib/types.ts	Added WhenPredicate type definition for custom fallback conditions
src/lib/index.ts	Modified exports to be more selective for Router module
src/lib/Router/Router.svelte	Added getRouterContextKey function and improved type imports
src/lib/Fallback/Fallback.svelte	Implemented when property with conditional rendering logic
src/lib/Fallback/Fallback.svelte.test.ts	Added comprehensive test suite covering all routing scenarios
src/testing/test-utils.ts	Added testing utilities for router setup and route manipulation
docs/testing-guide.md	Added comprehensive testing documentation

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