Add PowerShell language support

Implement PowerShell language server integration using PowerShell Editor
Services (PSES). The implementation includes:

- PowerShellLanguageServer class with automatic PSES download and setup
- Support for PowerShell file extensions: .ps1, .psm1, .psd1
- Symbol detection using PSES format: "function FuncName ()"
- 7 basic tests + 2 integration tests covering:
  - Document symbols and function detection
  - Within-file references
  - Cross-file go-to-definition
  - SerenaAgent find_symbol and find_references

Requires: pwsh (PowerShell 7+) in PATH. PSES auto-downloads on first use.

test/solidlsp/powershell/__init__.py

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+# PowerShell language server tests