Remove unnecessary types/vscode (fix build issue) (#6442)

Addresses this build issue:

```
[17:44:35] Error: @types/vscode ^1.73.0 greater than engines.vscode ^1.65.0. Consider upgrade engines.vscode or use an older @types/vscode version
    at validateVSCodeTypesCompatibility (/Users/jmcphers/git/positron/build/node_modules/@vscode/vsce/out/validation.js:115:19)
    at validateManifest (/Users/jmcphers/git/positron/build/node_modules/@vscode/vsce/out/package.js:913:59)
    at async Promise.all (index 0)
    at async listFiles (/Users/jmcphers/git/positron/build/node_modules/@vscode/vsce/out/package.js:1333:22)
```

The fix is to remove `@types/vscode` from positron-assistant and get
these types from Positron's internal copy instead.

extensions/positron-assistant/package.json

@@ -149,7 +149,6 @@
     "@openrouter/ai-sdk-provider": "^0.0.6",
     "@stylistic/eslint-plugin": "^2.9.0",
     "@types/node": "^20",
-    "@types/vscode": "^1.73.0",
     "ai": "^4.1.0",
     "eslint": "^9.13.0",
     "google-auth-library": "^9.15.1",

extensions/positron-assistant/tsconfig.json

@@ -17,7 +17,8 @@
 		"../../src/vscode-dts/vscode.proposed.mappedEditsProvider.d.ts",
 		"../../src/vscode-dts/vscode.proposed.defaultChat*.d.ts",
 		"../../src/vscode-dts/vscode.proposed.inlineCompletions*.d.ts",
+		"../../src/vscode-dts/vscode.d.ts",
 		"../../src/positron-dts/positron.d.ts",
 	],
-	"exclude": ["node_modules"]
+	"exclude": ["node_modules"],
 }