feat(pack): introduce plugin manifest support

Problem: There is no agreed way for plugins to provide extra information
  about themselves. Like required Neovim version, install/update/delete
  hooks, and dependencies.

Solution: Introduce basic concept of plugin manifest file `pkg.json`
  based on https://packspec.org/.

runtime/lua/vim/pack.lua

@@ -232,6 +232,25 @@
 ----- To act on install from lockfile, run before very first `vim.pack.add()`
 ---vim.api.nvim_create_autocmd('PackChanged', { callback = hooks })
 ---```
+---[vim.pack-manifest]()
+---
+---Plugins can come with a special top level `pkg.json` manifest file with extra
+---information. If present, `vim.pack` uses it for improved user experience:
+---- Running |:checkhealth| for `vim.pack` will perform extra checks to ensure
+---  healthy plugin installation.
+---
+---Full specification see at https://packspec.org/. See also |vim.pack.Manifest|.
+---A simple example:
+---```json
+---{
+---  "name": "best-plugin.nvim",
+---  "description": "The best plugin for Neovim",
+---  "engines": {
+---      "nvim": ">=0.13.0",
+---      "vim": ">=9.1.0"
+---  }
+---}
+---```
 
 local api = vim.api
 local uv = vim.uv
@@ -516,6 +535,37 @@ end
 local active_plugins = {}
 local n_active_plugins = 0
 
+--- @class vim.pack.ManifestEngines
+--- @inlinedoc
+--- @field nvim? string Version range for Nvim.
+--- @field vim? string Version range for Vim.
+
+--- @class vim.pack.Manifest
+--- @field name? string Plugin name
+--- @field description? string Plugin description
+--- Supported engine versions. Values should be |vim.version.range()| compatible specs.
+--- @field engines? vim.pack.ManifestEngines
+
+--- @param path string
+--- @return vim.pack.Manifest?
+local function manifest_read(path)
+  local manifest_path = vim.fs.joinpath(path, 'pkg.json')
+  local stat = uv.fs_stat(manifest_path)
+  if not stat then
+    return nil
+  end
+
+  local fd = uv.fs_open(manifest_path, 'r', 438)
+  if not fd then
+    return {}
+  end
+
+  local data = assert(uv.fs_read(fd, stat.size, 0))
+  assert(uv.fs_close(fd))
+  local ok, res = pcall(vim.json.decode, data)
+  return (ok and type(res) == 'table') and res or {}
+end
+
 --- @param plugs vim.pack.Plug[]
 --- @param event_name 'PackChangedPre'|'PackChanged'
 --- @param kind 'install'|'update'|'delete'
@@ -1468,6 +1518,8 @@ end
 --- @class vim.pack.PlugData
 --- @field active boolean Whether plugin was added via |vim.pack.add()| to current session.
 --- @field branches? string[] Available Git branches (first is default). Missing if `info=false`.
+--- Data from the |vim.pack-manifest|. Empty in case of reading error. Missing if `info=false`.
+--- @field manifest? vim.pack.Manifest
 --- @field path string Plugin's path on disk.
 --- @field rev string Current Git revision. Taken from |vim.pack-lockfile| if `info=false`.
 --- Git revision of a pending update. The same as used during |vim.pack.update()| and which
@@ -1494,6 +1546,7 @@ local function add_p_data_info(p_data_list, offline)
     funs[i] = function()
       p_data.branches = git_get_branches(path)
       p_data.tags = git_get_tags(path)
+      p_data.manifest = manifest_read(path)
 
       if not offline then
         git_fetch(path)