feat: add statusline function (#10)

Problem: no way to expose compiling/watching state to statusline
plugins like lualine or heirline without polling status() and
formatting it manually.

Solution: add `require('preview').statusline()` that returns
'compiling', 'watching', or '' for direct use in statusline components.

lua/preview/init.lua

@@ -41,6 +41,7 @@
 ---@field toggle fun(bufnr?: integer)
 ---@field open fun(bufnr?: integer)
 ---@field status fun(bufnr?: integer): preview.Status
+---@field statusline fun(bufnr?: integer): string
 ---@field get_config fun(): preview.Config
 local M = {}
 
@@ -188,6 +189,19 @@ function M.status(bufnr)
   return compiler.status(bufnr)
 end
 
+---@param bufnr? integer
+---@return string
+function M.statusline(bufnr)
+  bufnr = bufnr or vim.api.nvim_get_current_buf()
+  local s = compiler.status(bufnr)
+  if s.compiling then
+    return 'compiling'
+  elseif s.watching then
+    return 'watching'
+  end
+  return ''
+end
+
 M._test = {
   ---@diagnostic disable-next-line: assign-type-mismatch
   reset = function()