[Web] Implement disabling interactive selection

[loic-sharma]

This updates Flutter Web to ignore copy/paste/selections if enableInteractiveSelection is false in a text field.

Part of #157611

Scenarios tested

1. Click/tap on text: cursor should not move.
2. Double click/tap on text: text should not be selected
3. Triple click/tap on text: text should not be selected
4. Move cursor back (Left arrow): cursor should move back
5. Move cursor forward (Right arrow): cursor should move forward
6. Select previous word (Shift+Left arrow): text should not be selected, cursor moves to previous word
7. Select previous word (Shift+Right arrow): text should not be selected, cursor moves the next word
8. Select all shortcut (Ctrl+A or Apple+A): text should not be selected. ⚠️ With this fix, Flutter Web moves the cursor to the end of the text field.
9. Copy shortcut (Ctrl+C or Apple+C): clipboard should not be updated
10. Paste shortcut (Ctrl+V or Apple+V): clipboard should not be pasted
11. Right-click > Copy: clipboard should not be updated
12. Right-click > Paste: clipboard should not be pasted

Browsers tested

macOS: Chrome, Firefox, Safari

Example app

import 'package:flutter/material.dart';

void main() {
  runApp(
    MaterialApp(
      home: Scaffold(
        body: SafeArea(
          child: TextField(
            enableInteractiveSelection: false,
          ),
        ),
      ),
    ),
  );
}

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[mdebbar]

LGTM, thanks for fixing this!

Does this PR fully fix #157611 or is there something remaining?

[loic-sharma]

Does this PR fully fix #157611 or is there something remaining?

It fixes all the issues for Flutter Web, yup. Here's the corresponding PR that landed to fix Flutter native platforms: #171050. I'll close that issue once this PR lands :)