fix($markdown): support path without file extension when importing code snippets

[mitjat]

Summary

This PR fixes an issue where code snippets in markdown can reference a filename to include only if the filename has an extension.

For example, including a snippet like this:

<<< @/path/to/Dockerfile

(note the path does not contain a .) results in the snippet being rendered as the string Invalid code snippet option even if the target file exists.

The bug is caused by an overly restrictive regex that demands at least one literal . in the include path. This PR fixes the regex and adds a test case.

My personal preference would be to refactor the relatively large regex into simpler parsing steps, but I don't want to mess with it and inadvertently introduce subtle side effects :/

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Build-related changes
• Other, please describe:

If changing the UI of default theme, please provide the before/after screenshot: N/A

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications: N/A

The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)

You have tested in the following browsers: (Providing a detailed version will be better.)

• Chrome
• Firefox
• Safari
• Edge
• IE

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature
• Related documents have been updated
• Related tests have been updated

To avoid wasting your time, it's best to open a feature request issue first and wait for approval before working on it.

Other information:

[mitjat]

@kefranabg care to take a look when you have some time? I see you were the most recent reviewer for this module. Thanks!

[billyyyyy3320]

Please keep it, it would make sure import path with file extension works fine

[billyyyyy3320]

Sorry for the late review. Thanks