Tweak heuristics to find the project root.

The project root means "this is a folder containing node_modules/bs-platform/{platform}/bsc.exe".
This path needs to be correct because it drives the formatter.

The previous heuristics seemed to fail in the case users where using yarn workspaces.
Yarn workspaces seem to have the following layout:
```
/root
  /node_modules
  - package.json
  - yarn.lock
  /folder1
    /node_modules
    - package.json
    - bsconfig.json
```

The compiler seems to be located in the `node_modules` under the root and not in the `node_modules` of `folder1`.
By searching for `bscPartialPath` instead of the nearest `bsconfig.json`,
we can correctly determine the root.

server/src/server.ts

@@ -415,7 +415,7 @@ process.on("message", (msg: m.Message) => {
         if (projectRootPath == null) {
           let params: p.ShowMessageParams = {
             type: p.MessageType.Error,
-            message: `Cannot find a nearby ${c.bsconfigPartialPath}. It's needed for determining the project's root.`,
+            message: `Cannot find a nearby ${c.bscPartialPath}. It's needed for determining the project's root.`,
           };
           let response: m.NotificationMessage = {
             jsonrpc: c.jsonrpcVersion,

server/src/utils.ts

@@ -22,7 +22,7 @@ export let findProjectRootOfFile = (
 	source: p.DocumentUri
 ): null | p.DocumentUri => {
 	let dir = path.dirname(source);
-	if (fs.existsSync(path.join(dir, c.bsconfigPartialPath))) {
+	if (fs.existsSync(path.join(dir, c.bscPartialPath))) {
 		return dir;
 	} else {
 		if (dir === source) {