fix(@angular/cli): ignore unknown files when formatting schematic changes

[cexbrayat]

PR Checklist

Please check to confirm your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/angular/angular-cli/blob/main/CONTRIBUTING.md#-commit-message-guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• Other... Please describe:

What is the current behavior?

When schematics modify files such as .gitignore, Prettier cannot infer a parser and formatting fails.

npx prettier --check .gitignore
[error] No parser could be inferred for file ".gitignore".

What is the new behavior?

With ignore-unknown enabled, Prettier does not throw this parser error

Does this PR introduce a breaking change?

• Yes
• No

Other information

[alan-agius4]

This PR was merged into the repository. The changes were merged into the following branches:

• main: 4bbd8dc
• 21.2.x: d87dba6