chore: streamline common-assets styles

[namjul]

This PRs aim is to collect the scattered css styles back into common-all. Specifically the scss styles for dendron-plugin-views and nextjs-template. It adds two new scss files main-next.scss and main-plugin.scss that contain context specific styles and levels out the common styles only back into main.scss (and its dependend files).

The asset sync (distribution of the styles from common-all into views and next) is refactored to only be done from the dendron dev sync_assets cli command.

It also removes some unused code found during the implementation.

• [ ] document a workflow for editing styles

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[CLAassistant]

All committers have signed the CLA.

[kevinslin]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution. Samuel Hobl seems not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account. You have signed the CLA already but the status is still pending? Let us recheck it.

@namjul are you using a different email for this pr? if so, can you make sure to make an offering to the CLA bot?

[kevinslin]

can you rebase to resolve the conflicts?