Support Theia IDE

[RodogInfinite]

There's some difficulty getting rust-analyzer to function in Theia, see: eclipse-theia/theia#9136. This support, by extension, should also transfer to GitPod since it's built on Theia. For a yet undetermined reason, installing the extension when building doesn't work for me, but there are reports of it functioning when manually installing the .vsix and the binary, see: eclipse-theia/theia#7608, but it still gives the "Activating extension rust-analyzer failed: ..." notification.

Another thing to consider, Theia version is often behind the latest version of VSCode causing a version incompatibility with the latest releases of the extension. This can be currently be fixed in a hacky way, see: eclipse-theia/theia#8828.

[lnicola]

Theia seems to lag a lot behind VS Code, and we're using newer APIs for e.g. inlay hints. We're also using the pre-release version flag and platform-specific VSIX feature (which currently don't even work in OVSX). With Theia advertising the Code 1.53 API and us requiring 1.66, I don't think there's much we can do here.