Correct eslint command in lint script

The `lint` script of the "arduino-ide-extension" package is intended to use the ESLint linter tool to check for problems
in all the package's JavaScript and TypeScript code files. It is used by the continuous integration system to validate
contributions.

Previously, the command invoked `eslint` without any arguments. With the 8.x version of ESLint used by the project, it
is necessary to provide a path argument in order to cause it to lint the contents of files. Because that argument was
not provided, the script didn't do anything at all and so would return a 0 exit status even if the code had linting rule
violations.

This is fixed by adding a `.` path argument to the command invoked by the script. This will cause ESLint to recurse
through the `arduino-ide-extension` folder and lint the code in all relevant files.

arduino-ide-extension/package.json

@@ -13,7 +13,7 @@
     "download-ls": "node ./scripts/download-ls.js",
     "download-examples": "node ./scripts/download-examples.js",
     "generate-protocol": "node ./scripts/generate-protocol.js",
-    "lint": "eslint",
+    "lint": "eslint .",
     "prebuild": "rimraf lib",
     "build": "tsc",
     "build:dev": "yarn build",