Duplicate diagnostics when opening files, with paths containing / on Windows

[dimhatz]

First of all, a big thank you to the maintainers for this great plugin!

Steps to reproduce:

1. :cd myproject (optional, but convenient to test relative paths)
2. :e ~/myproject/src/file1.ts or use absolute path :e c:/myuser/myproject/src/file2.ts (do not use Tab completion, as it will change slashes to \)
3. modify the buffer to produce a ts error
4. lua =vim.diagnostics.get() will show duplicate entries of the error

This happens every time, when restoring nvim sessions, since the Session.vim file uses forward slashes.

NOTE: with relative paths (e.g. step 2 becomes :e src/file3.ts) this bug does not occur. It also does not occur when using \.

NOTE2: when a file is first opened with / and then closed, its /-containing path will be saved in :ls!. The following re-opening of the file, even with \ this time, will still result in the buggy behavior.

Additional info:

- Windows 10 x64
- nvim 0.10 (latest stable)
- typescript-tools latest, using only options: { separate_diagnostic_server = true, expose_as_code_action = 'all' }
(separate_diagnostic_server = false does not make any difference)
- lspconfig without mason-lspconfig
- tsserver: project-local from package [email protected]

Happy to provide more info if needed.