feat(healthcheck): support native lua healthchecks, and fixes

After neovim/neovim#15259 lua healthchecks are called directly and are
prefered over neovim autoload ones. The discover of Lua ones requires
the function to be named "check()".

Also the mentioned PR changed the design of healthchecks to not use
output capturing, therefore avoid printing and instead concatenate and
call the health functions.

.github/ISSUE_TEMPLATE/bug_report.yml

@@ -41,7 +41,7 @@ body:
 
   - type: textarea
     attributes:
-      label: Output of `:checkhealth nvim_treesitter`
+      label: Output of `:checkhealth nvim-treesitter`
       render: markdown
     validations:
       required: true