newrule(target_spacing_sufficient): WCAG 2.2: 2.5.8 Target size (minimum)

[shunguoy]

• New Rule(s): target_spacing_sufficient
• New or modified help files: target_spacing_sufficient.html

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

• Check WCAG 2.2 minimum target size  #1674
• <Help content> Check WCAG 2.2 minimum target size #1677
• [BUG] Formatting issue and typo in help for element_mouseevent_keyboard  #941

Additional information can be found here:

• WCAG 2.2 new rule: target_spacing_sufficient box note

Testing reference:

• Select "WCA_2_2" ruleset from the checker settings. then scan the following test case:
  ...test/v2/checker/accessibility/rules/target_spacing_sufficient_ruleunit/element_target.html

Note: due to a mapping issue (#1731), the 'Violation' is showing up as "Recommendation' now. "Undersized target "{0}" does not have sufficient spacing of 12 CSS pixels from another target "{1}"",

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.

[philljenkins]

good editorial changes

[ErickRenteria]

@shunguoy we have a few conflicts, please review.

[ErickRenteria]

Validated