feat: add support for polling a variable once at startup

[aspen-otr]

Description

For any defpoll variable, if :interval is zero, then its script is called exactly one time, and not scheduled for any future executions.

Usage

Simply define something such as (defpoll username :interval 0 "echo $USER") and instead of continuously calling the echo $USER script (as is current behavior) it will instead just store $USER inside username. This is probably preferred as $USER should not change while eww is running.

Additional Notes

While week-long intervals and similar strategies exist, this creates a standard way to execute a command once for a variable. Fixes #883.

Checklist

Please make sure you can check all the boxes that apply to this PR.

• All widgets I've added are correctly documented.
• I added my changes to CHANGELOG.md, if appropriate.
• The documentation in the docs/content/main directory has been adjusted to reflect my changes.
• I used cargo fmt to automatically format all code before committing