Format `MIRAI` diagnostic message

[seanrion]

Issue

I would like to suggest an enhancement to the diagnostic messages generated by MIRAI. Currently, MIRAI's diagnostic messages are formatted similarly to Rust's native diagnostic messages, which can make it challenging to differentiate between the two when reviewing output.

Expected Behavior

For example, a current MIRAI warning might look like this:

warning: possible attempt to add with overflow

To make it easier to identify messages originating from MIRAI, I propose adjusting the format to include a prefix like [MIRAI], resulting in:

warning: [MIRAI] possible attempt to add with overflow

This small change would greatly improve clarity and ease of use when analyzing diagnostic output, especially in mixed environments where both Rust and MIRAI diagnostics are present.