Bug report template: various tweaks

* Improve readability of reports by using headers.
* Improve readability of the versions list by using a table.
* Explicitly say that screenshots of code are not acceptable as a code sample.
* Added an "Install type" entry to the versions list
* Added an extra checklist for bug reporters in an attempt to reduce the number of issues which should never have been opened in the first place.

Author: jrfnl

.github/ISSUE_TEMPLATE/bug_report.md

@@ -18,23 +18,23 @@ Please report bugs for externally maintained sniffs to the appropriate external
 standard repository (not here).
 -->
 
-**Describe the bug**
+## Describe the bug
 A clear and concise description of what the bug is.
 
-**Code sample**
+### Code sample
 ```php
-echo "A short code snippet that can be used to reproduce the bug";
+echo "A short code snippet that can be used to reproduce the bug. Do NOT paste screenshots of code!";
 ```
 
-**Custom ruleset**
+### Custom ruleset
 ```xml
 <?xml version="1.0"?>
 <ruleset name="My Custom Standard">
   <description>If you are using a custom ruleset, please enter it here.</description>
 </ruleset>
 ```
 
-**To reproduce**
+### To reproduce
 Steps to reproduce the behavior:
 1. Create a file called `test.php` with the code sample above...
 2. Run `phpcs test.php ...`
@@ -43,14 +43,24 @@ Steps to reproduce the behavior:
 PHPCS output here
 ```
 
-**Expected behavior**
+## Expected behavior
 A clear and concise description of what you expected to happen.
 
-**Versions (please complete the following information):**
- - OS: [e.g., Windows 10, MacOS 10.15]
- - PHP: [e.g., 7.2, 7.4]
- - PHPCS: [e.g., 3.5.5, master]
- - Standard: [e.g., PSR2, PSR12, Squiz]
+## Versions (please complete the following information)
 
-**Additional context**
+|                         |                                                                              |
+|-------------------------|------------------------------------------------------------------------------|
+| Operating System        | [e.g., Windows 10, MacOS 10.15]                                              |
+| PHP version             | [e.g., 7.2, 7.4]                                                             |
+| PHP_CodeSniffer version | [e.g., 3.5.5, master]                                                        |
+| Standard                | [e.g., PSR2, PSR12, Squiz, custom]                                           |
+| Install type            | [e.g. Composer (global/local), PHAR, PEAR, git clone, other (please expand)] |
+
+## Additional context
 Add any other context about the problem here.
+
+## Please confirm:
+
+- [ ] I have searched the issue list and am not opening a duplicate issue.
+- [ ] I confirm that this bug is a bug in PHP_CodeSniffer and not in one of the external standards.
+- [ ] I have verified the issue still exists in the `master` branch of PHP_CodeSniffer.