fix(terminal): gf properly opens file (#45)

## Description
1. Currently `gf` only search in current working directory. Use `**` in
`findfile()` to do a recursive search. Otherwise, `gf` can't open a file
down a nested directory.
2. `vim.schedule` the command `vim.cmd("e ", f)`, because otherwise it
opens the file but goes to the buffer that was already opened. Not sure
why this happens, but if we schedule it correctly goes to the opened
buffer by `gf`.
3. Also changed `self:close()` to `self:toggle()`, so as not to reset
the state of the terminal unless a user specifically exits the terminal.
This is rather personal and also based on a [LazyVim
Discussion](https://github.com/LazyVim/LazyVim/discussions/4741), but I
can understand if it's not wanted in the scope of `snacks.nvim` and
probably users could change the behavior of the `gf` key in their
personal configuration.

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## Related Issue(s)
[A LazyVim
discussion](https://github.com/LazyVim/LazyVim/discussions/4741)
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  If this PR fixes any issues, please link to the issue here.
  - Fixes #<issue_number>
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## Screenshots

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lua/snacks/terminal.lua

@@ -27,12 +27,14 @@ Snacks.config.style("terminal", {
   wo = {},
   keys = {
     gf = function(self)
-      local f = vim.fn.findfile(vim.fn.expand("<cfile>"))
+      local f = vim.fn.findfile(vim.fn.expand("<cfile>"), "**")
       if f == "" then
         Snacks.notify.warn("No file under cursor")
       else
-        self:close()
-        vim.cmd("e " .. f)
+        self:hide()
+        vim.schedule(function()
+          vim.cmd("e " .. f)
+        end)
       end
     end,
     term_normal = {