Adjust repo config for VS Code formatting

[guidezpl]

If someone's user settings include the following, the current workspace level formatting settings don't apply since they aren't as specific.

"[dart]": {
  "editor.formatOnSave": true,
  "editor.formatOnType": true,
}

This PR handles this by turning off auto-formatting specifically for Dart files.

This PR also adds "files.trimTrailingWhitespace": true to simplify contribution guide.

Fixes #122050. This change is test-exempt.

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
• I signed the [CLA].
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is [test-exempt].
• All existing and new tests are passing.

[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption to this rule, contact Hixie on the #hackers channel in Chat (don't just cc him here, he won't see it! He's on Discord!).

If you are not sure if you need tests, consider this rule of thumb: the purpose of a test is to make sure someone doesn't accidentally revert the fix. Ask yourself, is there anything in your PR that you feel it is important we not accidentally revert back to how it was before your fix?

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[guidezpl]

@DanTup is my understanding of user vs. workspace settings + language-specific settings correct?

[DanTup]

Yes, you're right - I overlooked this in the last review 🙈

We should also add editor.formatOnPaste. That's what I meant to include in the comment at #122048 (comment) but apparently just included formatOnSave again.

[timsneath]

LGTM!

[Hixie]

test-exempt: configuration change

[timsneath]

@guidezpl I also noticed this:
https://github.com/flutter/flutter/wiki/Setting-up-the-Framework-development-environment#set-up-your-environment

which suggests the following config change:

    "files.trimTrailingWhitespace": true

If we can add that one also, I think we can delete the wiki VSCode configuration section, which will reduce additional configuration toil for contributors. Any chance you can add this to this PR?

[guidezpl]

If we can add that one also, I think we can delete the wiki VSCode configuration section, which will reduce additional configuration toil for contributors. Any chance you can add this to this PR?

Nice, I'll add it and edit the wiki.