Include ResolvedDiagnosticReason in -fdiagnostics-as-json

(for readability, all JSON is given using YAML syntax)

Motivation

-fdiagnostics-as-json provides GHC diagnostics in a machine readable form.

However, it is currently hard to reproduce the original GHC error/warning message from that.

For example given

version: "1.0"
ghcVersion: ghc-9.10.1
span:
  file: Foo.hs
  start: {line: 1, column: 18}
  end: {line: 1, column: 21}
severity: Error
code: 47854
message: ["`foo' is exported by `foo' and `foo'"]
hints: []

I would like to produce

Foo.hs:1:18: error: [GHC-47854] [-Wduplicate-exports, Werror=duplicate-exports]
    ‘foo’ is exported by ‘foo’ and ‘foo’

Proposal

Include reason in the output of -fdiagnostics-as-json, e.g. for the example from above include

reason: duplicate-exports

Addition to the JSON Schema:

reason:
  description: For warnings (and errors that were born as warnings) the name of the warning flag that triggered them
  type: string

Note that the type is string rather than string | null. Also note that reason is not a required field. This keeps existing messages conformant with the revised schema.