feat: Add stream-with-experimental-timestamps UI mode (#11538)
## Summary
Adds a new UI mode `stream-with-experimental-timestamps` that prints
timestamps before task labels in stream output. Timestamps are displayed
in gray using the format `[HH:MM:SS.mmm]`.
Example output:
```
[14:32:15.123] web:build: Building...
[14:32:15.456] api:build: Compiling...
[14:32:16.789] web:build: Build complete!
```
## Usage
Configure via `turbo.json`:
```json
{
"ui": "stream-with-experimental-timestamps"
}
```
Or via CLI flag:
```
turbo run build --ui=stream-with-experimental-timestamps
```
## Test Plan
- Added unit tests for timestamp formatting in `PrefixedUI` and
`PrefixedWriter`
- Added test case for parsing the new UI mode from turbo.json
- Verified CLI flag acceptance
