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/qf.lua

@@ -0,0 +1,74 @@
+local M = {}
+
+---Represents an item in a Neovim quickfix/loclist.
+---@class qf.item
+---@field bufnr? number The buffer number where the item originates.
+---@field filename? string
+---@field lnum number The start line number for the item.
+---@field end_lnum? number The end line number for the item.
+---@field pattern string A pattern related to the item. It can be a search pattern or any relevant string.
+---@field col? number The column number where the item starts.
+---@field end_col? number The column number where the item ends.
+---@field module? string Module information (if any) associated with the item.
+---@field nr? number A unique number or ID for the item.
+---@field text? string A description or message related to the item.
+---@field type? string The type of the item. E.g., "W" might stand for "Warning".
+---@field valid number A flag indicating if the item is valid (1) or not (0).
+---@field user_data? any Any user data associated with the item.
+---@field vcol? number Visual column number. Indicates if the column number is a visual column number (when set to 1) or a byte index (when set to 0).
+
+---@class snacks.picker
+---@field loclist fun(opts?: snacks.picker.Config): snacks.Picker
+---@field qflist fun(opts?: snacks.picker.Config): snacks.Picker
+
+---@class snacks.picker.qf.Config
+---@field qf_win? number
+---@field filter? snacks.picker.filter.Config
+
+local severities = {
+  E = vim.diagnostic.severity.ERROR,
+  W = vim.diagnostic.severity.WARN,
+  I = vim.diagnostic.severity.INFO,
+  H = vim.diagnostic.severity.HINT,
+  N = vim.diagnostic.severity.HINT,
+}
+
+---@param opts snacks.picker.qf.Config
+---@type snacks.picker.finder
+function M.qf(opts, filter)
+  local win = opts.qf_win
+  win = win == 0 and vim.api.nvim_get_current_win() or win
+
+  local list = win and vim.fn.getloclist(win, { all = true }) or vim.fn.getqflist({ all = true })
+  ---@cast list { items?: qf.item[] }?
+
+  local ret = {} ---@type snacks.picker.finder.Item[]
+
+  for _, item in pairs(list and list.items or {}) do
+    local row = item.lnum == 0 and 1 or item.lnum
+    local col = (item.col == 0 and 1 or item.col) - 1
+    local end_row = item.end_lnum == 0 and row or item.end_lnum
+    local end_col = item.end_col == 0 and col or (item.end_col - 1)
+
+    if item.valid == 1 then
+      local file = item.filename or item.bufnr and vim.api.nvim_buf_get_name(item.bufnr) or nil
+      local text = item.text or ""
+      ret[#ret + 1] = {
+        pos = { row, col },
+        end_pos = item.end_lnum ~= 0 and { end_row, end_col } or nil,
+        text = file .. " " .. text,
+        line = item.text,
+        file = file,
+        severity = severities[item.type] or 0,
+        buf = item.bufnr,
+        item = item,
+      }
+    elseif #ret > 0 and ret[#ret].item.text and item.text then
+      ret[#ret].item.text = ret[#ret].item.text .. "\n" .. item.text
+      ret[#ret].item.line = ret[#ret].item.line .. "\n" .. item.text
+    end
+  end
+  return filter:filter(ret)
+end
+
+return M