Allow several paths for 'fortran-ls.executablePath'

[phil-blain]

Hi @hansec ! :)

I'm using this extension over Remote-SSH.
I'm configuring the path to fortls in the Remote-SSH config file, i.e. $HOME/.vscode-server/data/Machine/settings.json.
I'm also tracking this file in my dotfiles repo to sync my settings across machines, so it would be great if the setting would support a list of paths that the extension looks for ther language server in. This way if a path is not available on some machine it can go on to the next path. This is the way I set it up for git.path, which accepts an array.

This would not be necessary if environment variables like $HOME would be supported in VScode settings (microsoft/vscode#2809).