accessibility support issue template (#2270)

Author: WilcoFiers

.github/ISSUE_TEMPLATE/ACCESSIBILITY_SUPPORT.md

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+## Accessibility support
+
+(( Put the accessibility support item here ))
+
+## ACT Rules
+
+- ... List the rules here, link to its acc support section
+
+## Test cases
+
+### Test case 1 ...
+
+(( Add test case here ))
+
+```html
+code snippet here
+```
+
+[Open this test case]()
+
+(( Add a link to a code pen or other hosted test case. Keep in mind this test case needs to be accessible and persistent. ))
+
+#### Preconditions
+
+Before running the test, check your settings. In many cases default settings aren't strictly required and will not impact the results. Use common sense here. If non-default settings are necessary for you, record the settings used in the reported results.
+
+- Ensure browsers are in the default settings, turn off unnecessary extensions
+- Ensure ATs are in the default settings
+- Ensure operating system are in default settings
+- Ensure software uses the correct version (usually the latest version)
+
+#### Test instructions
+
+(( Provide instructions on how to test the test case, this should include: ))
+
+- Which technologies to use
+- Any setting changes necessary to run the test
+- Any variation in how to operate the different assistive technologies
+- Step by step instructions on how to check the test case
+- What the expected results are
+
+### External links
+
+(( List any external links to bugs in browsers & AT, WPT, or spec issues. Be sure to open issues in the appropriate repositories. ))
+
+<!--
+Links to external resources to look for / raise issues
+- Web platform tests: https://github.com/web-platform-tests/wpt
+- JAWS issues: https://github.com/FreedomScientific/VFO-standards-support/issues
+- NVDA issues: https://github.com/nvaccess/nvda/issues
+- Firefox issues: https://bugzilla.mozilla.org/
+- Chromium issues: https://issues.chromium.org/issues?q=componentid:1363614+
+- Safari issues: https://github.com/WebKit/webkit/issues
+- ARIA: https://github.com/w3c/aria/issues
+- HTML: https://github.com/whatwg/html/issues
+- CSS: https://github.com/w3c/csswg-drafts/issues
+- AccName: https://github.com/w3c/accname/issues
+- HTML AAM: https://github.com/w3c/html-aam/issues
+- ARIA in HTML: https://github.com/w3c/html-aria/issues
+- PowerMapper tests: https://www.powermapper.com/tests/
+- Accessibility support: https://www.a11ysupport.io/
+-->
+
+## Test results
+
+Please report test results in a comment below. Please use the following format:
+
+```md
+## Test case X
+
+- Date: ...
+- Operating system: ..., version ...
+- Browser: ..., version ...
+- Assistive technology: ..., version ...
+- Test result: Passed / Failed (if failed, how was it different from expected?)
+```