Improve Bash heredoc highlighting (#28185)

Release Notes:

  - Improved Bash heredoc highlighting

| Zed 0.180.2 | With this PR |
| --- | --- |
|
![Image](https://github.com/user-attachments/assets/aa2534af-53df-4f01-988e-f18ec52a2b62)
|
![Image](https://github.com/user-attachments/assets/8fc92113-41f2-4249-ab81-6beb0a1469ca)
|

```bash
cat << EOT >> hello.txt
hello world
EOT
```

- `<<`: `operator`
- `EOT`: `string`
0708d476cachbk committed on 4/6/2025, 3:14:05 PM· committed by GitHubparent57669b4
1 file changedLine totals unavailable