feat: add support for preserving comments in code (#967)

Summary:
Changelog:
* **[Feature]**: Preserve comments in unminified builds, while continuing to strip all comments from minified builds.

Add support for preserving comments in code. Useful for preserving `/*#__PURE__*/` annotations that aid tree shaking.

NOTE: In a future version of Metro, the default minifier options will change to align with the underlying minifier's default, e.g. `comments: 'some'` in Terser.

Pull Request resolved: #967

Test Plan:
Test case here: microsoft/rnx-kit#2365

|        |    Size |
| :----- | ------: |
| Before | 1567764 |
| After  | 1258413 |
| Diff   | -309351 |

Reviewed By: jacdebug

Differential Revision: D45223689

Pulled By: motiz88

fbshipit-source-id: 89c1e4050e67c3c34ec74015986827425b5fc76c

packages/metro-config/src/__tests__/__snapshots__/loadConfig-test.js.snap

@@ -131,6 +131,7 @@ Object {
       },
       "output": Object {
         "ascii_only": true,
+        "comments": false,
         "quote_style": 3,
         "wrap_iife": true,
       },
@@ -308,6 +309,7 @@ Object {
       },
       "output": Object {
         "ascii_only": true,
+        "comments": false,
         "quote_style": 3,
         "wrap_iife": true,
       },
@@ -485,6 +487,7 @@ Object {
       },
       "output": Object {
         "ascii_only": true,
+        "comments": false,
         "quote_style": 3,
         "wrap_iife": true,
       },
@@ -662,6 +665,7 @@ Object {
       },
       "output": Object {
         "ascii_only": true,
+        "comments": false,
         "quote_style": 3,
         "wrap_iife": true,
       },

packages/metro-config/src/defaults/index.js

@@ -110,6 +110,7 @@ const getDefaultValues = (projectRoot: ?string): ConfigT => ({
       },
       output: {
         ascii_only: true,
+        comments: false,
         quote_style: 3,
         wrap_iife: true,
       },