Fix invalid json for max warnings option

[ybiquitous]

I think stylelint should output any error or warning messages to STDERR instead of STDOUT.
Because users should expect that piping is available.

For example, here is an example of using the jq command:

$ stylelint --formatter=json --max-warnings=0 a.css | jq .

Max warnings exceeded: 1 found. 0 allowed

[
  {
    "source": "/tmp/a.css",
    "deprecations": [],
    "invalidOptionWarnings": [],
    "parseErrors": [],
    "errored": false,
    "warnings": [
      {
        "line": 1,
        "column": 2,
        "rule": "block-no-empty",
        "severity": "warning",
        "text": "Unexpected empty block (block-no-empty)"
      }
    ]
  }
]

Which issue, if any, is this issue related to?

Fix #5265

Is there anything in the PR that needs further explanation?

In addition, I have inserted EOL at the beginning of the warning message so that users can easily read the entire output.
But this change may be out of the scope of this PR. I want feedback, if any.
(I can revert the insertion if anyone opposites it.)

Before:

$ stylelint -f json --max-warnings=0 a.css
[{"source":"/Users/masafumi.koba/git/stylelint/stylelint/a.css","deprecations":[],"invalidOptionWarnings":[],"parseErrors":[],"errored":false,"warnings":[{"line":1,"column":2,"rule":"block-no-empty","severity":"warning","text":"Unexpected empty block (block-no-empty)"}]}]Max warnings exceeded: 1 found. 0 allowed

After:

$ stylelint -f json --max-warnings=0 a.css
[{"source":"/Users/masafumi.koba/git/stylelint/stylelint/a.css","deprecations":[],"invalidOptionWarnings":[],"parseErrors":[],"errored":false,"warnings":[{"line":1,"column":2,"rule":"block-no-empty","severity":"warning","text":"Unexpected empty block (block-no-empty)"}]}]
Max warnings exceeded: 1 found. 0 allowed

---

Note: I will merge the change of #5266 into this PR if approved.

[jeddy3]

Super, thanks.

All looks good to me, including the EOL insertion.

[ybiquitous]

@jeddy3 Thanks for the review! I've merged the #5266 change into this branch.

[mattxwang]

LGTM, and good added test!

[jeddy3]

• Fixed: invalid JSON for max-warnings option (#5267).