Allow the OS to handle custom URIs. Cannot open links with custom URL schema (e.g. slack://)

Does this issue occur when all extensions are disabled?: Yes/No Yes

• VS Code Version: 1.60.1
• OS Version: All

Steps to Reproduce:

1. Obtain a custom URI. For Mac users an easy way to test a custom URI is with Hook from a file or email. Or you could create a bearbones app with a custom URI with a few lines of AppleScript. These types of links are common e.g. slack://, Zoom, OmniFocus, and several more so you should be able to find a URI that works. You can test these in your browser.
2. Add the link to a markdown file open in VSCode
3. Try to click it in the code editor and markdown preview. Observe the error.

Additional Info

Poking around I see vscode only handles a limited list. If a schema is unknown, VSCode must delegate to the OS to handle it. It also occurs to me that mailto doesn’t require special treatment on this list and should be handled by the OS.

https://github.com/microsoft/vscode/blob/021071ff74a53038dae299db2fe1328beed13da0/extensions/markdown-language-features/src/util/links.ts#L9-L16

This was reported before and fell through the cracks. The same console errors where VSCode attempts to resolve a markdown file with a filename that is a substring of the url it does not understand:

Testing custom scheme:
[p.ideas.test.md](hook://file/3NE0H471r?p=RGVuZHJvbi92YXVsdA==&n=p.ideas.test.md)

Unable to open '3NE0H471r.md': Unable to read file '/Users/brandon/Dendron/vault/hook:/file/3NE0H471r.md'
(Error: Unable to resolve non-existing file '/Users/brandon/Dendron/vault/hook:/file/3NE0H471r.md').

https://github.com/microsoft/vscode/issues/104429

[Stedag]

I did a PR to the Dendron Extension which provides a tidy workaround for this issue via dendron: Open link.

[AlexAtkinson]

Whatever you’re doing with this, please just simply add ‘tel’ href links to the sanitization list as requested here: https://github.com/microsoft/vscode/issues/133365