Enable opening a single Terraform file

[jpogran]

This PR enables single file support by skipping configuring the file watcher with information about what folders to watch. This leaves the watcher with little environment information, but still provides enough functionality to edit a single file. This suits the common transitory workflows for single files. For example, opening a file just to review contents or to make a few line changes without needing the context from an entire Terraform project.

A warning is shown to the user when a single file is opened, explaining that some capabilities may be reduced when editing a single file, and to open a directory to regain full editor functionality. This is to warn the user that while we enable this workflow, we don't encourage it because of the tradeoffs we currently have to do. This warning can be disabled by setting ignoreSingleFileWarning to true.

[radeksimko]

I gave it a try and it works pretty well! 👍🏻

I left some naming suggestions in-line and there are now some conflicts to resolve after merging #839

Enqueueing a new directory was

walker.EnqueuePath(modPath)

and is now

stateStore.WalkerPaths.EnqueueDir(dirUri)

i.e. the enqueueing happens on the state store (memdb) as opposed to the walker itself.

Secondly there are now two walkers (svc.closedDirWalker & svc.openDirWalker) so some setup methods previously called on svc.walker now need to be called on both instead.

I am happy to assist with the rebasing and resolution of conflicts, if you like - feel free to ping me and I can jump on Zoom with you!

[radeksimko]

One remaining error handling + commented out code, otherwise LGTM

[radeksimko]

Just proposed a small change to the warning message, but LGTM otherwise 🚀

Tested in combination with hashicorp/vscode-terraform#1031 and it works great!

[github-actions]

This functionality has been released in v0.27.0 of the language server.

For further feature requests or bug reports with this functionality, please create a new GitHub issue following the template. Thank you!

[github-actions]

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