feat(services): add PlainServicesApi facade - W-22419571

[peternhale]

Summary

• Adds a Promise-based PlainServicesApi facade to salesforcedx-vscode-services that allows external consumers to use all services without importing or depending on effect
• Exposes connection, workspace, project, settings, config, aliases, file system, editor, channel, metadata, source tracking, templates, trace flags, terminal, and component set operations as simple Promise-returning methods
• Bridges reactive state (SubscriptionRef, PubSub) to standard vscode.Event<T> patterns
• Existing Effect-based services property remains unchanged (non-breaking addition)
• Updates READMEs for both salesforcedx-vscode-services and salesforcedx-vscode-services-types to document the new plain API as the recommended consumption pattern

Test plan

• 42 unit tests covering all facade methods, error handling, and reactive event bridging
• Full monorepo compilation passes (38 packages)
• Full monorepo lint passes (77 scripts)
• Full monorepo test suite passes (159 tests in services package)
• Manual verification: consumer extension can call api.getConnection() without any effect imports

@W-22419571@

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