JS code for setting up button disabling/enabling

Proper setup of button disabling/enabling based on conditions.

https://freelancehunt.com/project/nuzhen-chelovek-so-znaniem/1019579.html

If, upon page load, the length of the fieldset is greater than or equal to 0, the button becomes disabled.
If fieldset is being added to the page, the button becomes active. Additionally, if, during the removal of fieldset, it reaches a count of 1, disable the button for removing them.