Integration of LSP Server to unify Shopware Language Support across IDEs

[shyim]

User Story: Unified Shopware Language Support Across IDEs

As a Shopware dev, I want to be able to use language intelligence features in the Shopware Language Server, so that I can reuse them consistently across multiple IDEs.

Acceptance Criteria:

• PhpStorm plugin integrates via the Language Server Protocol.
• Existing language features are powered by shopware-lsp instead of custom plugin logic.
• Plugin structure follows JetBrains LSP integration guidelines.
• No feature regressions compared to the current plugin state.
• Language-related logic is implemented in shopware-lsp, not IDE-specific code.
• Shared features work identically across supported IDEs.
• IDE-specific plugins act primarily as thin LSP clients.
• New language features are added only in the LSP repository.
• All reusable functionality resides in the Shopware Language Server
• New language intelligence features are rejected in the plugin if they belong in the LSP.
• Contribution guidelines reflect the LSP-first strategy.
• Plugin scope is limited to LSP integration and IDE-specific configuration only.

Connected to shopware/shopware-cli#872 (because that functionality is also in phpstorm)

Context

We started last year of implementing a Official Shopware Language Server: https://github.com/shopwareLabs/shopware-lsp and we're making really progress there and therefore it makes now really sense to migrate the PhpStorm plugin too to the same structure. Therefore we can share the code and the features between multiple IDEs.

See https://plugins.jetbrains.com/docs/intellij/language-server-protocol.html

Therefore we're not implementing any new Feature here anymore, which can be implemented in the LSP itself.