feat(jsx-email): improve preview speed, support file create and delete again

[shellscape]

Component / Package Name:

This PR contains:

• bugfix
• feature
• refactor
• documentation
• other

Are tests included?

• yes (bugfixes and features will not be merged without tests)
• no

Breaking Changes?

• yes (breaking changes will not be merged unless absolutely necessary)
• no

If yes, please include "BREAKING CHANGES:" in the first commit message body, followed by a description of what is breaking.

List any relevant issue numbers:

Description

This PR does a bunch:

• re-enables template file creation and deletion in the preview app
• speeds up the preview initialization and hmr substantially
• preview app uses the hash router now, to ease burden for deployed preview apps
• removes the dependency on @jsx-email/app-preview. the preview app is now copied to the package before publishing. the value of having a separate package was slowly disappearing, and with the latest change it became clear that we had to do version checking from jsx-email to make sure the preview app package was on a minimum version. that would have created more burden for users. the package size purists won't be happy with the increase in size of the main package, especially those not using the preview, but overall it's a better DX