Add textDocument/rename support

[Jason-Abbott]

Implement rename refactoring to rename variables and templates across files.

Rename targets:

• Variables: Rename $varName and all references within scope
• Templates: Rename template definition and all {{template "name"}} calls

Implementation notes:

• For variables: Scope-aware rename (only within declaring scope)
• For templates: Cross-file rename using WorkspaceTemplateManager
• Validate new name doesn't conflict with existing symbols
• Use prepareRename to validate rename is possible at cursor

LSP methods:

• textDocument/rename - perform the rename, return WorkspaceEdit
• textDocument/prepareRename - validate rename is possible, return range