Add template_interpolation validator

Implement a new provider::validatefx::template_interpolation validator.

Description:

• Validate Terraform interpolation syntax (e.g. ${var.foo}).

Acceptance criteria:

1. Subtask: Implement template_interpolation validator in internal/validators
2. Subtask: Add unit tests for template_interpolation validator
3. Subtask: Add template_interpolation function to internal/functions package
4. Subtask: Add unit tests for template_interpolation function
5. Subtask: Register template_interpolation in registry.go
6. Subtask: Add documentation in examples/functions/template_interpolation/function.tf
7. Subtask: Update docs table using make docs command
8. Subtask: Add integration test in integration/main.tf