feat: add subcommand to force-poll variable

[tokinanpa]

Description

This PR adds a subcommand to the CLI to manually poll a polling variable:

eww poll [NAMES]...

This command will force the variable to be polled outside of its interval. It will also ignore :run-while conditions.
An error will be thrown if the variable isn't defined with defpoll (or if it doesn't exist, obviously).

Resolves #1005

Additional Notes

This is my first time contributing, and I haven't been able to fully ensure that this code works as intended. Suggestions and bug fixes would be greatly appreciated.

Checklist

• I added my changes to CHANGELOG.md.
• 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

[tokinanpa]

1. currently, the command silently ignores non-existent variables, same if no variables are given at all.

Strange, I thought I handled the case where the variable doesn't exist. I'll have to look at that again.

[tokinanpa]

@w-lfchen As far as I can tell, the behavior of eww update and eww poll are exactly the same here. Neither print an error message to stderr when called with a nonexistent variable, they both put an error message in the logs. The update subcommand also does nothing when called with no arguments.
Is there something I'm missing?

[w-lfchen]

Is there something I'm missing?

my bad, i was a bit tired and mixed up eww update and eww get.
i still think that both shouldn't silently fail but copying eww update's behavior is the correct thing to do here imo.

[w-lfchen]

this looks good, thank you for your work! <3