Update @react-email/markdown to use md-to-react-email 5.x

[tmcw]

Goals

1. In 2023, the new thing to show up in my node_modules making it big is duplicate esbuild versions
2. react-email includes its own version of esbuild
3. By keeping react-email out of my build dependencies, I can avoid another copy of esbuild
4. But, @react-email/markdown depends on md-to-react-email, which until 5.0.0, depends on react-email, which depends on esbuild, hence an esbuild

Background

Esbuild is big, and @react-email's components are lovely and small, except for this accidental dependency change

Proposal

Update @react-email/markdown's dependencies to keep it from inviting another esbuild copy into node_modules.