Adding echo argument to wp_terms_checklist() to allow return of generated markup

[kevinlangleyjr]

Currently, wp_terms_checklist() echos the output with no way to just return the generated markup without opening an output buffer and capturing the output. In this patch, I've added an extra argument to wp_terms_checklist() to allow the return of the markup.

[kevinlangleyjr]

Adding echo arg to wp_terms_checklist() function to allow returning of the generated markup or echo by default

[boonebgorges]

Bonus: this will make it easier to write unit tests.

[boonebgorges]

In 33904:

Allow wp_terms_checklist() to return markup rather than echoing it.

Props kevinlangleyjr.
Fixes #33720.