[mlir-vscode] Fix processing of files not within the workspace

In a previous commit we added proper support for separate configurations
per workspace folder, but that effectively broke support for processing out-of-workspace
files. Given how useful this is (e.g. when iterating on a test case in /tmp), this
commit refactors server creation to support this again. We support this case using
a "fallback" server that specifically handles files not within the workspace. This uses
the configuration settings for the current workspace itself (not the specific folder).

Differential Revision: https://reviews.llvm.org/D123183