This is the first part of #39018. One of the common things for new users
coming from more dynamic languages like JavaScript, Python or Ruby is to
use `+` to concatenate strings. However, this doesn't work that way in
Rust unless the first type is a `String`. This commit adds a check for
this use case and outputs a new error as well as a suggestion to guide
the user towards the desired behavior. It also adds a new test case to
test the output of the error.
