Shim/file system: fix leaks of the shim

[auduchinok]

Fixes the file system shim is leaked after a solution is closed:

[github-actions]

❗ Release notes required

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✅ Found changes and release notes in following paths:

Change path	Release notes path	Description
src/Compiler	docs/release-notes/.FSharp.Compiler.Service/9.0.200.md

[auduchinok]

We can also hide the mutable shim behind a property, so capturing the getter wouldn't be a problem.

[T-Gro]

Just for my understanding, did you search for all usages of " >> FileSystem" or something similar?

[T-Gro]

We can also hide the mutable shim behind a property, so capturing the getter wouldn't be a problem.

I like this encapsulation proposal more.
Avoid also future incorrect usages, not just the existing ones.

[auduchinok]

Just for my understanding, did you search for all usages of " >> FileSystem" or something similar?

Yes, I've looked through all the usages.