Use annotate-snippets for emitting errors

[bash]

Resolves #2729.

There are a couple of things that I'm not sure if I did them correctly:

• annotate-snippets doesn't have an "internal error" annotation type. I'm currently printing "internal" as the annotation id (where the error id usually is in rustc):
  
• Does rustfmt have any guarantees about the public API when used as a library? Because I removed fancy_print and the Display impl from FormatReport both of which were pub.
• Is there a particular reason that "warning" was previously printed in red? It is now printed in yellow:
  

[scampi]

Thanks for your contribution ! Can you add few docs about the new functions you added ?

Regarding fancy_print I believe its targeted use was within the crate. If used as a library, I think one would rely on the report struct, not on its pretty output (unless for debugging).
However, given we reached 1.0, we should strive for not breaking public API. @topecongiro what do you think ? What about marking that method as deprecated ?

[topecongiro]

@scampi
There is an RFC which defines rustfmt stability (cc rust-lang/rust#54504). In the RFC, API braeking change is defined as follows:

A change that could cause a dependent crate not to build, could break a script using the executable, or breaks specification-level formatting compatibility. A formatting change in this category would be frustrating even for occasional users.

and this includes changes to the public API to rustfmt as a library. Hence, removing fancy_print and the Display impl from FormatReport is a breaking change and requires a major version update.

[topecongiro]

@bash This looks great, thank you! I really like the change you brought with this PR, so using them internally is perfectly fine. But for the reason I noted in the above comment, could you put back the removed fancy_print and Display impl for FormatReport?

[scampi]

@topecongiro thanks for pointing this out

[bash]

@scampi
Thanks for the review! I addressed both of your code comments.
I still need to improve/add comments to the new functions added in this PR.

[bash]

@topecongiro
I re-implemented fancy_print and the Display impl for FormatReport using the new FormatReportFormatter. I've marked both as deprecated, because I want to encourage consumers of the library to choose the new API. Is that a viable solution?

[scampi]

LGTM! Are you OK with the changes @topecongiro ?

[topecongiro]

Awesome, thank you!