Add Regal language server support

This is WIP but getting there. There are a few changes here:

* The extension has been extended to support one or more language servers for Rego. Currently only [Regal](https://github.com/StyraInc/regal). These are started based on configuration and can also be installed if the user desires.
* The OPA installation logic and downloader has been made generic to support fetching of regal and other binaries as needed in future.
* Language servers are promoted to users if they are not installed.

Some suggested follow on work:

* make binary dependency checking generic, and use server module for OPA checking
* make finding of binary paths more generic

TODOs still include:

- [ ] testing of this change on a windows machine
- [ ] testing of this change on a linux machine
- [ ] Updating Regal and sharing a patch release with a minor LS change (0.16.1) (StyraInc/regal#539)

Signed-off-by: Charlie Egan <[email protected]>

.vscode/extensions.json

@@ -1,7 +1 @@
-{
-	// See http://go.microsoft.com/fwlink/?LinkId=827846
-	// for the documentation about the extensions.json format
-	"recommendations": [
-		"eg2.tslint"
-	]
-}
+{}

.vscode/launch.json

@@ -3,7 +3,7 @@
 // Hover to view descriptions of existing attributes.
 // For more information, visit: https://go.microsoft.com/fwlink/?linkid=830387
 {
-	"version": "0.2.0",
+    "version": "0.2.0",
     "configurations": [
         {
             "name": "Extension",
@@ -16,7 +16,11 @@
             "outFiles": [
                 "${workspaceFolder}/out/**/*.js"
             ],
-            "preLaunchTask": "npm: watch"
+            "autoAttachChildProcesses": true,
+            "preLaunchTask": {
+                "type": "npm",
+                "script": "watch"
+            }
         },
         {
             "name": "Extension Tests",