Add the VS Code MLIR extension to the Open VSX extension registry

[rdong8]

In a similar vein to #93544, please add the MLIR extension to Open VSX so that it can be used in VS Code compatible IDEs like VSCodium, Cursor and Windsurf.

[llvmbot]

@llvm/issue-subscribers-mlir

Author: None (rdong8)

In a similar vein to https://github.com//issues/93544, please add the [MLIR extension](https://marketplace.visualstudio.com/items?itemName=llvm-vs-code-extensions.vscode-mlir) to [Open VSX](https://open-vsx.org/) so that it can be used in VS Code compatible IDEs like VSCodium, Cursor and Windsurf.