newrule(draggable_alternative_exists): WCAG 2.2: 2.5.7 Dragging Movements

[shunguoy]

• New Rule(s): draggable_alternative_exists
• New or modified help files: draggable_alternative_exists

This PR is related to the following issue(s):

• Check WCAG 2.2 drag movement alternative exists #1675
• <Help content> Check WCAG 2.2 drag movement alternative exists #1724

Additional information can be found here:

• box note for WCAG 2.2 new rule: draggable_alternative_exists

Testing reference:

first, select WCAG_2_2 from the ruleset list
then scan test/v2/checker/accessibility/rules/draggable_alternative_exists_ruleunit/element_dropstop.html
You should see an NR: "Ensure the draggable element "img" has a "single pointer" alternative" in the result.

I have conducted the following for this PR:

• I validated this code in Chrome and FF
• I validated this fix in my local env
• I provided details for testing
• This PR has been reviewed and is ready for test
• I understand that the title of this PR will be used for the next release notes.

[ErickRenteria]

Validated: