add healthcheck

[znd4]

Hi, I wasn't sure if the way I'd installed jsregexp was working, so I've tried to implement a :checkhealth. Let me

I'm not sure if I've formatted the messages properly. Also, on my machine the require("luasnip.util.jsregexp") raises an exception instead of returning false, but I don't think that that's an issue unless luasnip needs more than one check in the future.

[znd4]

~~FWIW, I've tried to install jsregexp with home-manager's extraLuaPackages, and this is what I see in the new checkhealth entry:~~

EDIT: nevermind, the healthcheck worked once I switched to master

[L3MON4D3]

Hey, that's pretty cool, thank you! (the failing tests are unrelated to these changes ;) )

[mathjiajia]

since jsregexp is optional, it is better to replace error with warning.

[L3MON4D3]

Ay, thanks for the suggestions, good points :+1: I'll amend them :) See 79cc25c3