Implement optional svelte-hmr support

[Valexr]

rixo/svelte-hmr - https://github.com/rixo/svelte-hmr
intrnl/vite-plugin-svelte - https://github.com/intrnl/vite-plugin-svelte/blob/trunk/lib/index.js (like example)

[EMH333]

Hmm, I think that would be very helpful! I'm a bit overwhelmed at the moment so this will have to go on the backlog for now. You are welcome to open a PR!

[Valexr]

Svelte-HMR have a many operations in JS... think it’s may doing esbuild slowed..🤔 & I’m not sure how it’s implementing properly... but may be I will try 🤓

[EMH333]

I'm actually going to reopen this issue because I want to track people's interest. When I have time, this would be a good weekend project, if you don't get to it first.

[Valexr]

👌🏻Optional HMR may be good 🤓

[alexanderniebuhr]

Would be great to have. Sadly, do not have free time atm too. but could help with it

[EMH333]

In the interest of heading towards a v1.0.0 release for this plugin, I'm going to remove this as a blocker for that milestone because esbuild doesn't support HMR fundamentals at the moment (see evanw/esbuild#97). I'll continue to leave this issue open since it looks like this is a popular request. Once the upstream support is there, I can take another look. I took a very very very hacky stab at HMR runtime support and it didn't seem too crazy.

[wathhr]

I think HMR would be a worthwhile addition as, in my opinion, this is whatis holding esbuild back the most. HMR would be a great addition to have for DX, just look at vite. This could be useful to take a look at on how this would be implemented: https://github.com/remorses/bundless/blob/master/plugins/svelte/src/index.ts