tech(LS): re-organise language-server library (#1555)

- Encapsulated core logic inside of `/lib/`, `MessageHandler.ts` functions as the entry point here.
  - Tests have been updated to use the new `lib/xyz` import

- Stray top-level files have been categorised
  - `*util` files have been removed in favour of more explicitly named files.
  - `/ast/` defines the logic for navigating the schema
  - `completions` has been broken up into constituents for the various sections of schema
  - `panic`, `wasm`, and `WasmPanicRegistry` were all previously separate top-level files. To reflect their coupled relation with `prisma-schema-wasm`, they have now been moved into it's new sub-dir: `error`

contributes https://github.com/prisma/team-orm/issues/367

Author: Druue

packages/language-server/src/__test__/artificial-panic.test.ts

@@ -5,7 +5,7 @@ import {
   handleCompletionRequest,
   handleDiagnosticsRequest,
   handleDocumentFormatting,
-} from '../MessageHandler'
+} from '../lib/MessageHandler'
 import { CURSOR_CHARACTER, findCursorPosition, getTextDocument } from './helper'
 
 import * as assert from 'assert'

packages/language-server/src/__test__/completion.test.ts

@@ -1,5 +1,5 @@
 import { TextDocument } from 'vscode-languageserver-textdocument'
-import { handleCompletionRequest } from '../MessageHandler'
+import { handleCompletionRequest } from '../lib/MessageHandler'
 import { CompletionList, CompletionParams, CompletionItemKind, CompletionTriggerKind } from 'vscode-languageserver'
 import assert from 'assert'
 import dedent from 'ts-dedent'