feat: add detach provider field and vim.g.preview config support (#42)

Problem: viewer processes launched via a string[] `open` command were
always killed on buffer deletion with no way to opt out. Configuring
the plugin also required an explicit `setup()` call in a `config`
hook, preventing config from being declared before the plugin loads.

Solution: add a `detach` boolean to `ProviderConfig` that skips
SIGTERM on buffer unload. Auto-call `setup()` from `vim.g.preview`
at module load time, enabling config via lazy.nvim's `init` hook.
Update vimdoc and README accordingly.

README.md

@@ -21,8 +21,18 @@ Typst, Markdown, etc.)—diagnostics included.
 
 ## Installation
 
-Install with your package manager of choice or via
-[luarocks](https://luarocks.org/modules/barrettruth/preview.nvim):
+With lazy.nvim:
+
+```lua
+{
+  'barrettruth/preview.nvim',
+  init = function()
+    vim.g.preview = { typst = true, latex = true }
+  end,
+}
+```
+
+Or via [luarocks](https://luarocks.org/modules/barrettruth/preview.nvim):
 
 ```
 luarocks install preview.nvim