Add ability to explicitly specify root modules as a workspace setting

[paultyng]

Users' should be able to manually override root modules for a given Terraform workspace or folder. The config should be scoped resource.

"terraform.rootModules": [
  "dir1",
  "dir2/dir3",
  "otherDir"
]

The paths should be relative to the root of the folders in the workspace (not sure if we want to allow for initialization outside workspace scope, maybe as a follow-on issue to investigate).

Given the above config and the following tree structure for a single workspace folder:

.
├── dir1
│   └── main.tf 
├── someDir
│       └── main.tf 
└── dir2
    ├── notRoot
    └── dir3
        └── main.tf 

The extension should enumerate all the directories in the setting that match actual directories present and pass those to the LS. In this case it would be:

[
  "dir1",
  "dir2/dir3"
]

(You'll notice otherDir is not passed as it doesn't match anything on disk, and someDir is not passed as its not in the settings).

---

Optionally we may also want to support per root module expanded configuration.

"terraform.rootModules": {
  "dir1": {
    "someSetting": ""
  }, 
  "dir2/dir3": {
    "someOtherSetting": ""
  }
}

[ghost]

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