feat(transformer): replace tab-to-space hack with native indent configuration (#14191)

## Summary
- Closes oxc-project/backlog#150
- Replaces manual tab-to-space replacement hack with native indentation configuration

## Changes
- Configure `Driver::codegen_options()` to use spaces with width 2
- Configure `TestCase` Codegen instance to use spaces with width 2
- Remove three instances of `cow_replace("\t", "  ")` hack
- Add `IndentChar` imports where needed

## Context
The `CodegenOptions` struct already had `indent_char` and `indent_width` fields available. This change leverages existing infrastructure to provide cleaner, more maintainable code instead of post-processing the output with string replacement.

This addresses the issue mentioned in oxc-project/backlog#150 where transformer conformance output was replacing tabs with spaces as a workaround.

🤖 Generated with [Claude Code](https://claude.ai/code)

Author: Boshen

.github/workflows/ci.yml

@@ -256,7 +256,6 @@ jobs:
               - '!npm/**'
               - '!crates/oxc_linter/**'
               - '!crates/oxc_language_server/**'
-              - '!tasks/**'
 
       - uses: ./.github/actions/clone-submodules
         if: steps.filter.outputs.src == 'true'