[FEATURE] defvar parameter to reset variable on window close

[fabolous005]

Description of the requested feature

When changing a variable with eww update foo=bar it will stay like this until the next update.
Sometimes you may want the variable to go back to it's initial value before the window opens again.

Proposed configuration syntax

(defvar foo :reset true "bar")

Additional context

My current workaround is to update the variable whenever i close the window, but that becomes a lot of boilerplate and overhead for multiple variables.

[fabolous005]

A maybe common usecase is to reset the index for a stack widget...