Add .tmp.cfg to top-level gitignore

[colin-grant-work]

What it does

Adds .tmp.cfg to the root .gitignore so that the temporary config directory created by the Playwright theia:start script is ignored regardless of where it lands.

Prior to the yarn-to-npm migration in #14481 (0036769), the script used yarn --cwd ../browser start, which didn't change the shell's working directory. $PWD still pointed at examples/playwright/, so .tmp.cfg was created there and covered by examples/playwright/.gitignore.

The npm migration replaced yarn --cwd with cd ../browser && npm run start, which shifts $PWD to examples/browser/ before $PWD is evaluated. A subsequent fix in #16455 (c895e74) moved rimraf after the cd so cleanup and creation were consistent, but both now target examples/browser/ — leaving an untracked .tmp.cfg that the existing .gitignore doesn't cover.

Rather than reworking the script (npm has no cross-platform equivalent of yarn --cwd), this PR simply adds .tmp.cfg to the root .gitignore.

How to test

1. Run npm run test:playwright from the repository root (or npm run ui-tests from examples/playwright/).
2. Verify that git status no longer shows an untracked examples/browser/.tmp.cfg/ entry.

Follow-ups

The examples/playwright/.gitignore entry for .tmp.cfg is now redundant but harmless. It could be removed in a follow-up.

Breaking changes

• This PR introduces breaking changes and requires careful review. If yes, the breaking changes section in the changelog has been updated.

Attribution

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines
• User-facing text is internationalized using the nls service (for details, please see the Internationalization/Localization section in the Coding Guidelines)

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[ndoschek]

Thank you @colin-grant-work, LGTM 👍