feat: Proper floating window for keymap help and OutlineStatus

No more obnoxious '}' on the cmdline when pressing `?`!

scope:
- More type hints
- Added class Float for creating floating windows with size that fit the
  content and position centered on the screen
- show_help action for outline window (key `?`) now uses a floating
  window
- :OutlineStatus now provides better information, and shows content in a
  floating window.

future:
- Floating window option configuration

lua/outline/float.lua

@@ -0,0 +1,89 @@
+-- local cfg = require('outline.config')
+
+local Float = {}
+
+---@class outline.Float
+---@field bufnr integer
+---@field winnr integer
+---@field ns integer
+
+function Float:new()
+  return setmetatable({ bufnr = nil, winnr = nil, ns = nil }, { __index = Float })
+end
+
+---@param lines string[]
+---@param hl outline.HL[]
+---@param title string
+---@param indent integer?
+function Float:open(lines, hl, title, indent)
+  indent = indent or 0
+
+  self.bufnr = vim.api.nvim_create_buf(false, true)
+  vim.api.nvim_buf_set_option(self.bufnr, 'bufhidden', 'delete')
+
+  local maxwidth = 0
+  for _, l in ipairs(lines) do
+    if #l > maxwidth then
+      maxwidth = #l
+    end
+  end
+
+  local ui = vim.api.nvim_list_uis()[1]
+  local nvim_height, nvim_width = ui.height, ui.width
+
+  local padding_w = 3
+
+  local height, width = math.min(nvim_height, #lines + 1), maxwidth + 2 * padding_w
+  local row = math.floor((nvim_height - height) / 2)
+  local col = math.floor((nvim_width - width) / 2)
+
+  self.winnr = vim.api.nvim_open_win(self.bufnr, true, {
+    relative = 'editor',
+    width = width,
+    height = height,
+    row = row,
+    col = col,
+    border = 'rounded',
+    style = 'minimal',
+    title = title,
+    title_pos = 'center',
+  })
+
+  if indent > 0 then
+    local pad = string.rep(' ', indent)
+    for i = 1, #lines do
+      lines[i] = pad .. lines[i]
+    end
+  end
+
+  vim.api.nvim_win_set_option(self.winnr, 'winfixwidth', true)
+  vim.api.nvim_buf_set_lines(self.bufnr, 0, -1, false, lines)
+  vim.api.nvim_buf_set_option(self.bufnr, 'modifiable', false)
+  vim.api.nvim_buf_set_option(self.bufnr, 'ft', 'OutlineHelp')
+
+  if hl then
+    self.ns = vim.api.nvim_create_namespace('OutlineHelp')
+    for _, h in ipairs(hl) do
+      vim.api.nvim_buf_add_highlight(
+        self.bufnr,
+        self.ns,
+        h.name,
+        h.line,
+        h.from + indent,
+        (h.to ~= -1 and h.to + indent) or -1
+      )
+    end
+    vim.api.nvim_win_set_hl_ns(self.winnr, self.ns)
+  end
+end
+
+function Float:close()
+  if self.winnr then
+    vim.api.nvim_buf_clear_namespace(self.bufnr, self.ns, 0, -1)
+    vim.api.nvim_win_close(self.winnr, true)
+    self.winnr = nil
+    self.bufnr = nil
+  end
+end
+
+return Float