feat: add markdown output format for lint command

[lornajane]

What/Why/How?

Related to #1539, add markdown format output to the lint command, similar to what we have for stats. This could be used in a lint status report, a github job summary, or anywhere else where a renderable format is useful.

Testing

Caution: PR doesn't include tests yet, I'd like to get some feedback on the feature.

Screenshots (optional)

Markdown output example:

validating /home/lorna/temp/datasette.yaml...
# Lint: ../../temp/datasette.yaml

| Severity | Location | Problem | Message |
|---|---|---|---|
| error | line 18:5 | [security-defined](https://redocly.com/docs/cli/rules/security-defined/) | Every operation should have security defined on it or on the root level. |
| warning | line 5:1 | [info-license](https://redocly.com/docs/cli/rules/info-license/) | Info object should contain `license` field. |

/home/lorna/temp/datasette.yaml: validated in 11ms

❌ Validation failed with 1 error and 1 warning.
run `redocly lint --generate-ignore-file` to add all problems to the ignore file.

---

validating /home/lorna/temp/datasette.yaml...

Lint: ../../temp/datasette.yaml

Severity	Location	Problem	Message
error	line 18:5	security-defined	Every operation should have security defined on it or on the root level.
warning	line 5:1	info-license	Info object should contain license field.

/home/lorna/temp/datasette.yaml: validated in 11ms

❌ Validation failed with 1 error and 1 warning. run redocly lint --generate-ignore-file to add all problems to the ignore file.

---

Check yourself

• [ ] Code changed? - Tested with redoc/reference-docs/workflows (internal)
• [ ] All new/updated code is covered with tests
• [ ] New package installed? - Tested in different environments (browser/node)

Security

• [ ] Security impact of change has been considered
• [ ] Code follows company security practices and guidelines

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: 39b8eac11e526a0cab9577e6558b49a98ce7e842

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

Name	Type
@redocly/openapi-core	Minor
@redocly/cli	Minor

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[github-actions[bot]]

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
redocly lint packages/core/src/benchmark/benches/rebilly.yaml	961.9 ± 16.4	936.6	983.7	1.00
redocly-next lint packages/core/src/benchmark/benches/rebilly.yaml	987.8 ± 33.3	946.6	1068.8	1.03 ± 0.04

[github-actions[bot]]

Coverage report

St.:grey_question:	Category	Percentage	Covered / Total
🟡	Statements	77.23%	4481/5802
🟡	Branches	67.51%	2468/3656
🟡	Functions	70.86%	744/1050
🟡	Lines	77.44%	4215/5443

Test suite run success

735 tests passing in 102 suites.

Report generated by 🧪jest coverage report action from 39b8eac11e526a0cab9577e6558b49a98ce7e842

[lornajane]

Other changes that I think we should consider for this feature:

• less "clutter" in the output, so the content can easily be piped to a file and reused in another context?
• emoji :warning: :stop_sign: or something
• something else?