Add option to enable/disable .cabal file support

Support for `.cabal` files was implemented in HLS 2.0.0.0.
But the vscode-haskell extension does not enable this Language Server
support, because it would break for older HLS binaries, as these were
unable to handle file updates for `.cabal` files.
To work-around, we had a pre-release of the vscode-haskell plugin, which
enabled support for `.cabal` plugins in an opt-in fashion.

It's been a while, and we want to enable language server support for
`.cabal` files unconditionally. There are still HLS binaries around that
can't handle `.cabal` files, but it much less.
For the unfortunate souls left behind with GHC versions such as 8.6.5,
we add a new vscode-haskell option:

    haskell.supportCabalFiles

This option is `true` by default, but if set to `false` it will
unconditionally **disable** sending File Notifications for `.cabal`
files, which allows us to keep working for older HLS binaries.

This change would be difficult to test, and incur an, in my opinion,
unreasonable price on our CI times.
I manually tested the changes, by using HLS 1.8.0.0 (last released
binary for GHC 8.6.5) on a project with `haskell.supportCabalFiles` set
to `true` and `false` respectively.
HLS seemed to behave correctly, showing Diagnostics for Haskell files,
but not show any logs for `.cabal` files.
The only potential change in behaviour is that vscode-haskell now also
activates when a `.cabal` file has been opened. This may be overly eager to
some, but since no Haskell files are typechecked, the impact on memory
usage is negligible in my opinion.

Author: fendor

package.json

@@ -234,6 +234,12 @@
             "Prefer a multiple component session, if the build tool supports it. At the moment, only `cabal` supports multiple components session loading. If the `cabal` version does not support loading multiple components at once, we gracefully fall back to \"singleComponent\" mode."
           ]
         },
+        "haskell.supportCabalFiles": {
+          "scope": "resource",
+          "default": true,
+          "type": "boolean",
+          "description": "Enable Language Server support for `.cabal` files. Requires Haskell Language Server version >= 2.0.0.0."
+        },
         "haskell.maxCompletions": {
           "scope": "resource",
           "default": 40,

src/extension.ts

@@ -205,6 +205,9 @@ async function activateServerForFolder(context: ExtensionContext, uri: Uri, fold
     args = args.concat(extraArgs.split(' '));
   }
 
+  const cabalFileSupport: boolean = workspace.getConfiguration('haskell', uri).supportCabalFiles;
+  logger.info(`Support for '.cabal' files enabled: ${cabalFileSupport ? 'yes' : 'no'}`);
+
   // If we're operating on a standalone file (i.e. not in a folder) then we need
   // to launch the server in a reasonable current directory. Otherwise the cradle
   // guessing logic in hie-bios will be wrong!
@@ -253,14 +256,21 @@ async function activateServerForFolder(context: ExtensionContext, uri: Uri, fold
 
   const pat = folder ? `${folder.uri.fsPath}/**/*` : '**/*';
   logger.log(`document selector patten: ${pat}`);
+
+
+  const cabalDocumentSelector = cabalFileSupport ? [{ scheme: 'file', language: 'cabal', pattern: pat }] : [];
+  const haskellDocumentSelector = [
+    { scheme: 'file', language: 'haskell', pattern: pat },
+    { scheme: 'file', language: 'literate haskell', pattern: pat },
+  ];
+
   const clientOptions: LanguageClientOptions = {
     // Use the document selector to only notify the LSP on files inside the folder
     // path for the specific workspace.
-    documentSelector: [
-      { scheme: 'file', language: 'haskell', pattern: pat },
-      { scheme: 'file', language: 'literate haskell', pattern: pat },
-      { scheme: 'file', language: 'cabal', pattern: pat },
-    ],
+    documentSelector:
+      [ ... haskellDocumentSelector
+      , ... cabalDocumentSelector
+      ],
     synchronize: {
       // Synchronize the setting section 'haskell' to the server.
       configurationSection: 'haskell',