Adopt and enforce new `WP_CLI_CS` standard

[jrfnl]

We have a new PHPCS standard for WP-CLI called WPCliCS. It is part of the wp-cli/wp-cli-tests package starting with version v2.1.0.

To adopt & enforce this new standard, the following actions need to be taken for this repository:

• [ ] Create a PR that adds a custom ruleset phpcs.xml.dist to the repository
  • Add phpcs.xml.dist file
  • Adapt .distignore to ignore phpcs.xml.dist & phpunit.xml.dist
  • Adapt .gitignore to ignore phpunit.xml, phpcs.xml & .phpcs.xml
  • Require version ^2.1 of the wp-cli/wp-cli-tests as a dev dependency
• [ ] Make any required changes to the code that fail the checks from the above ruleset in separate PRs
• [ ] Merge the ruleset once all required changes have been processed and merged

A sample PR for a simple repository can be seen here: https://github.com/wp-cli/maintenance-mode-command/pull/3/files

Related wp-cli/wp-cli#5179

[jrfnl]

I've done the prep work to implement this and am left with 9 issues for which I'd like the opinion of one of the maintainers of this package on how to solve this. /cc @schlessera

• 5 issues related to whether or not some (protected, not private) properties should be safe to rename.
• 4 issues about strict comparisons - I know how to change it, but can't easily determine whether these would be safe to change without tracing through the code extensively, which is why I'm asking if someone has a quick answer.

FILE: inc\checks\class-check.php
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FOUND 0 ERRORS AND 3 WARNINGS AFFECTING 3 LINES
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 14 | WARNING | Property name "$_when" should not be prefixed with an underscore to
    |         | indicate visibility (PSR2.Classes.PropertyDeclaration.Underscore)
 23 | WARNING | Property name "$_status" should not be prefixed with an underscore to
    |         | indicate visibility (PSR2.Classes.PropertyDeclaration.Underscore)
 30 | WARNING | Property name "$_message" should not be prefixed with an underscore to
    |         | indicate visibility (PSR2.Classes.PropertyDeclaration.Underscore)
------------------------------------------------------------------------------------------

FILE: inc\checks\class-file-contents.php
------------------------------------------------------------------------------------------
FOUND 0 ERRORS AND 2 WARNINGS AFFECTING 2 LINES
------------------------------------------------------------------------------------------
 34 | WARNING | Found: ==. Use strict comparisons (=== or !==).
    |         | (WordPress.PHP.StrictComparisons.LooseComparison)
 49 | WARNING | Found: ==. Use strict comparisons (=== or !==).
    |         | (WordPress.PHP.StrictComparisons.LooseComparison)
------------------------------------------------------------------------------------------

FILE: inc\checks\class-file.php
------------------------------------------------------------------------------------------
FOUND 0 ERRORS AND 2 WARNINGS AFFECTING 2 LINES
------------------------------------------------------------------------------------------
 15 | WARNING | Property name "$_when" should not be prefixed with an underscore to
    |         | indicate visibility (PSR2.Classes.PropertyDeclaration.Underscore)
 47 | WARNING | Property name "$_matches" should not be prefixed with an underscore to
    |         | indicate visibility (PSR2.Classes.PropertyDeclaration.Underscore)
------------------------------------------------------------------------------------------

FILE: inc\checks\class-option-value.php
------------------------------------------------------------------------------------------
FOUND 0 ERRORS AND 2 WARNINGS AFFECTING 2 LINES
------------------------------------------------------------------------------------------
 44 | WARNING | Found: ==. Use strict comparisons (=== or !==).
    |         | (WordPress.PHP.StrictComparisons.LooseComparison)
 52 | WARNING | Found: ==. Use strict comparisons (=== or !==).
    |         | (WordPress.PHP.StrictComparisons.LooseComparison)
------------------------------------------------------------------------------------------