feat(snacks): added snacks.picker (#445)

## Description

More info coming tomorrow.

In short:
- very fast. pretty much realtime filtering/sorting in huge repos (like
1.7 million files)
- extensible
- easy to customize the layout (and lots of presets) with
`snacks.layout`
- simple to create custom pickers
- `vim.ui.select`
- lots of builtin pickers
- uses treesitter highlighting wherever it makes sense
- fast lua fuzzy matcher which supports the [fzf
syntax](https://junegunn.github.io/fzf/search-syntax/) and additionally
supports field filters, like `file:lua$ 'function`

There's no snacks picker command, just use lua.

```lua
-- all pickers
Snacks.picker()

-- run files picker
Snacks.picker.files(opts)
Snacks.picker.pick("files", opts)
Snacks.picker.pick({source = "files", ...})
```

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maintainers
  why we should accept this pull request. -->

## Todo
- [x] issue with preview loc not always correct when scrolling fast in
list (probably due to `snacks.scroll`)
- [x] `grep` (`live_grep`) is sometimes too fast in large repos and can
impact ui rendering. Not very noticeable, but something I want to look
at.
- [x] docs
- [x] treesitter highlights are broken. Messed something up somewhere

## Related Issue(s)

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  If this PR fixes any issues, please link to the issue here.
  - Fixes #<issue_number>
-->

## Screenshots

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lua/snacks/dashboard.lua

@@ -749,13 +749,18 @@ end
 function M.pick(cmd, opts)
   cmd = cmd or "files"
   local config = Snacks.config.get("dashboard", defaults, opts)
+  local picker = Snacks.picker.config.get()
   -- stylua: ignore
   local try = {
     function() return config.preset.pick(cmd, opts) end,
     function() return require("fzf-lua")[cmd](opts) end,
     function() return require("telescope.builtin")[cmd == "files" and "find_files" or cmd](opts) end,
     function() return require("mini.pick").builtin[cmd](opts) end,
+    function() return Snacks.picker(cmd, opts) end,
   }
+  if picker.enabled then
+    table.insert(try, 1, table.remove(try, #try))
+  end
   for _, fn in ipairs(try) do
     if pcall(fn) then
       return