fix: shim require for ESM

[sapphi-red]

Sorry, #18 wasn't working because require doesn't work in ESM.
This PR shim require for ESM.

[antfu]

Can we directly create the require function in place? I am not sure the shim is needed

[sapphi-red]

I forgot that tsup will shim import.meta.url. (I had to add esbuildOptions.supported though)
I updated the PR.

[sapphi-red]

import-meta not supported error was happening because the target was ES2017.
Because vite-plugin-vue-markdown depends on Vite 2+ that supports Node 12+, I changed the target to node12 instead of setting esbuildOptions.supported in the latest commit. node12 is ES2019 + some features according to https://node.green/ .