bug: `fortls` relative path resolution is unsuccessful

[gnikit]

In pre-release 3.4.0 the added local path resolution is buggy. e.g.

With

• "fortran.fortls.path": "venv/bin/fortls"
• "fortran.fortls.path": "./venv/bin/fortls"
• "fortran.fortls.path": "./venv/bin/"
• "fortran.fortls.path": "venv/bin/"
  I have a message that said that fortls is not installed

And with "fortran.fortls.path": "${workspaceFolder}/venv/bin/fortls" I have a new message:

and the following lines in the logs:

[ERROR - 1:57:25 PM] [lsp.client] Unable to launch LS to check version:
[ERROR - 1:57:25 PM] spawnSync /Data/WORK/Projets/arbalete/${workspaceFolder}/venv/bin/fortls ENOENT
Error: spawnSync /Data/WORK/Projets/arbalete/${workspaceFolder}/venv/bin/fortls ENOENT
	at Object.spawnSync (node:internal/child_process:1111:20)
	at spawnSync (node:child_process:834:24)
	at n.FortlsClient.getLSVersion (/home/pouxa/.vscode/extensions/fortran-lang.linter-gfortran-3.4.0/dist/extension.js:2:184404)
	at n.FortlsClient.didOpenTextDocument (/home/pouxa/.vscode/extensions/fortran-lang.linter-gfortran-3.4.0/dist/extension.js:2:181204)
	at process.processTicksAndRejections (node:internal/process/task_queues:96:5)

/Data/WORK/Projets/arbalete being my workspaceFolder

Originally posted by @pums974 in #656 (comment)