fix(image): skip \usepackage in comments and body (#2325)

## Description

Here are some small and hopefully uncontroversial tweaks to package
extraction from LaTeX preambles:

* Don't consider anything in comments
* Make sure that the extracted names are actually the arguments of
`\usepackage` and not some other macro on the same line
* Stop looking for packages at `\begin{document}`, where the preamble
ends and the body of the document begins (you can't load packages after
this, so any `\usepackage` beyond this point is content, not code. Also
saves a huge amount of work in large documents.)

Co-authored-by: Folke Lemaitre <folke.lemaitre@gmail.com>

lua/snacks/image/doc.lua

@@ -128,12 +128,15 @@ function M.get_packages(buf)
   return M._cache(buf, "packages", function()
     local ret = {} ---@type string[]
     for _, line in ipairs(vim.api.nvim_buf_get_lines(buf, 0, -1, false)) do
+      line = line:match("(.-)%%") or line
       if line:find("\\usepackage", 1, true) then
-        for _, p in ipairs(vim.split(line:match("{(.-)}") or "", ",%s*")) do
+        for _, p in ipairs(vim.split(line:match("\\usepackage.-{(.-)}") or "", ",%s*")) do
           if not vim.tbl_contains(ret, p) then
             ret[#ret + 1] = p
           end
         end
+      elseif line:find("\\begin{document}", 1, true) then
+        break
       end
     end
     return ret