feat(webpack): allow watching node_modules

[rigor789]

PR Checklist

• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/tools/notes/CONTRIBUTING.md#commit-messages.
• There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
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• All existing tests are passing: https://github.com/NativeScript/NativeScript/blob/master/tools/notes/DevelopmentWorkflow.md#running-unit-tests-application.
• Tests for the changes are included - https://github.com/NativeScript/NativeScript/blob/master/tools/notes/WritingUnitTests.md.

What is the current behavior?

What is the new behavior?

Passing --env.watchNodeModules, or adding the following to the webpack.config.js:

const webpack = require("@nativescript/webpack");

module.exports = (env) => {
  // enable watching node_modules
  env.watchNodeModules = true;
  
  webpack.init(env);
  
  // Learn how to customize:
  // https://docs.nativescript.org/webpack
  
  return webpack.resolveConfig();
};

Will trigger a rebuild when something in node_modules changes. This is disabled by default as in most cases node_modules shouldn't change, however this is useful when debugging a plugin/core etc right in node_modules.