Support regex search for email templates in dev server command

[midanosi]

Goals

1. Regex search for template files within a specified folder

Background

Scenario:
I have a react-email template file, called SendWeeklyDigest.tsx. I have another file called sendWeeklyDigest.ts, which fetches some data, generates the email body via the template, then sends the email.

I would like to group my concerns by function execution. So those two files both live in a folder digests. This isn't

This isn't possible at the moment, as the only way to store react-email template files in a repository (as I understand it), is by having one single folder that is only allowed to contain react-email templates.

Proposal

Add --include option to dev server command that allows specifying where my react-email templates live.

And then suggesting files as templates, e.g. SendWeeklyDigest.template.tsx or SendWeeklyDigest.template.tsx

Then the dev server command would be email dev --include src/email/*.template.tsx --port 3003

[gabrielmfern]

We did already think of this kind of thing and have a solution implemented for it, just haven't quite got around to documenting it.
Here's a part from a PR I'm writing that touches a bit on this:

react-email/apps/docs/cli.mdx

Lines 86 to 103 in 7cbf829

	### How to make the preview server ignore directories?
	Once the preview server has started and is now open on `localhost`, the preview server
	reads recursively down into all of your files and directories. This can be disabled
	from a directory down by prefixing it with `_`, e.g. `components -> _components`. So if you wanted
	to make components for your emails, you could make a structure as follows:
	```bash
	my-project
	├── emails
	│ ├── _components
	│ │ └── this-is-not-going-to-appear-in-the-sidebar.tsx
	│ ├── email.tsx
	│ └── static
	├── package.json
	└── tsconfig.json
	```