Use absolute paths for dotnet launched DLLs

[FelixDamrau]

resolves #88

Zed spawns dotnet with its working directory set to the worktree root.
We must use absolute paths to reference the cs-language server, so the extension installed one can be found.

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[FelixDamrau]

info

Only affects systems that do not have cs-language server installed as a global tool.
Guess that's why it wasn't found before/during development.
@rbstp -> Can you confirm that?

Same problem is in roslyn.rs, but this usually doesn't start DLLs

[FelixDamrau]

@cla-bot check

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[rbstp]

@FelixDamrau Turns out I've got csharp-ls installed as a global dotnet tool, so worktree.which("csharp-ls") finds it and short-circuits before we ever reach the dotnet exec path. That's why this never showed up while I was working on it.

I went back and reproduced the failure using the relative DLL path from a cwd outside the extension, and confirmed that the absolute path resolves it fine.