Markdown Language Features (built-in) version 1.44.2 report an error in Theia

[lmcbout]

Bug Description:

When you upgrade the Markdown Language Features (built-in) version from version 1.39.1 to version 1.44.2, opening a README.md file does not show

An error is reported:
failed: n.window.registerCustomEditorProvider is not a function.

Steps to Reproduce:

1. Update package,json with
   FROM
   "vscode-builtin-markdown-language-features": "https://open-vsx.org/api/vscode/markdown-language-features/1.39.1/file/vscode.markdown-language-features-1.39.1.vsix",
   TO
   "vscode-builtin-markdown-language-features": "https://open-vsx.org/api/vscode/markdown-language-features/1.44.2/file/vscode.markdown-language-features-1.44.2.vsix",
2. Remove the plugins folder
   3.yarn download:plugins
3. Start Theia with a workspace pointing to Theia
4. In the explorer view, locate a README.md file
   Select Context menu/Open With/ Preview
   --> DO NOT WORK with builtin 1.44.2
   --> work with builtin 1.39.1

Additional Information

• Operating System: UBUNTU 18.04
• Theia Version: LATEST

[vince-fugnitto]

@lmcbout the API has been recently marked as final and is not yet supported: #6636

[vince-fugnitto]

I tested with 1.50.0 of the extension and it works well given that registerCustomEditorProvider is implemented.