add slash to namespace wp functions

[circlecube]

Proposed changes

Type of Change

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)

Video

Checklist

• I have read the CONTRIBUTING doc
• I have viewed my change in a web-browser
• Linting and tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Further comments

[circlecube]

This fixes an issue where we were sometimes seeing in testing: Fatal error: Uncaught Error: Call to undefined function NewfoldLabs\WP\Module\Data\wp_is...

It hasn't been reported in any live sites, but fixing here to avoid that.