`mock-fs` is incompatible with Node v20

[jmsjtu]

Summary

The mock-fs package is incompatible with Node v20, which causes unit tests in the aura-language-server and lightning-lsp-common to fail.

It looks like there's a draft PR in mock-fs to support Node v20 but progress seems to be slow.

A potential replacement is memfs, although there's some extra plumbing that needs to happen to make it compatible with existing tests.

Steps To Reproduce:

1. Install node v20
2. Run unit tests

Expected result

No test failures

Actual result

Test failures

[jmsjtu]

The fix introduced in #593 is a temporary work-around to unblock other PRs.