Improve UART.

[floitsch]

Adds some Toit extensions.

[github-actions]

	Warnings
⚠️	Some issues found for the commit messages in this PR: the commit message "Improve UART.": summary should not end with a period (full stop) summary looks empty type/action looks empty Please fix these commit messages - here are some basic tips: follow Conventional Commits style correct format of commit message should be: <type/action>(<scope/component>): <summary>, for example fix(esp32): Fixed startup timeout issue allowed types are: change,ci,docs,feat,fix,refactor,remove,revert,test sufficiently descriptive message summary should be between 20 to 72 characters and start with upper case letter avoid Jira references in commit messages (unavailable/irrelevant for our customers) TIP: Install pre-commit hooks and run this check when committing (uses the Conventional Precommit Linter).
⚠️	The Pull Request description looks very brief, please check if more details can be added.
⚠️	The **target branch** for this Pull Request **must be the default branch** of the project (`master`). If you would like to add this feature to a different branch, please state this in the PR description and we will consider it.

👋 Hello floitsch, we appreciate your contribution to this project!

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[kasperl]

Looks good to me.