Handle single chars with to_string() for single_char_add_str

[belyakov-am]

Add support for single chars / literals with to_string() call for push_str() and insert_str().

changelog: [single_char_add_str]: handle single chars with to_string() call

Closes #12775

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @xFrednet (or someone else) some time within the next two weeks.

Please see the contribution instructions for more information. Namely, in order to ensure the minimum review times lag, PR authors and assigned reviewers should ensure that the review label (S-waiting-on-review and S-waiting-on-author) stays updated, invoking these commands when appropriate:

• @rustbot author: the review is finished, PR author should check the comments and take action accordingly
• @rustbot review: the author is ready for a review, this PR will be queued again in the reviewer's queue

[xFrednet]

Welcome to Clippy. A fantastic first contribution. Everything looks good to me.

[xFrednet]

Bors our merge bot,
as something to say,
to merge this PR,
it will test it hurray.

This is just a fun way, to add this PR into the merge queue :P

[bors]

📌 Commit f7723ca has been approved by xFrednet

It is now in the queue for this repository.

[belyakov-am]

Thank you!
I'm glad to contribute to Clippy. Looking forward to more contributions in the future

[bors]

⌛ Testing commit f7723ca with merge 87c895a...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: xFrednet
Pushing 87c895a to master...