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/picker/source/meta.lua

@@ -0,0 +1,46 @@
+local M = {}
+
+---@class snacks.picker
+---@field pickers fun(opts?: snacks.picker.Config): snacks.Picker
+
+---@param file string
+---@param t table<string,unknown>
+function M.table(file, t)
+  file = Snacks.meta.file(file)
+  local values = vim.tbl_keys(t)
+  table.sort(values)
+  ---@param value string
+  return vim.tbl_map(function(value)
+    return {
+      file = file,
+      text = value,
+      search = ("/^M\\.%s = \\|function M\\.%s("):format(value, value),
+    }
+  end, values)
+end
+
+---@param opts snacks.picker.Config
+---@type snacks.picker.finder
+function M.pickers(opts)
+  return M.table("picker/config/sources.lua", opts.sources or {})
+end
+
+---@param opts snacks.picker.Config
+---@type snacks.picker.finder
+function M.layouts(opts)
+  return M.table("picker/config/layouts.lua", opts.layouts or {})
+end
+
+function M.actions()
+  return M.table("picker/actions.lua", require("snacks.picker.actions"))
+end
+
+function M.preview()
+  return M.table("picker/preview.lua", require("snacks.picker.preview"))
+end
+
+function M.format()
+  return M.table("picker/format.lua", require("snacks.picker.format"))
+end
+
+return M