feat(plugin): dev.path can now be a function (#1157)

In some case, `dev.path .. plugin.name` is not enoguh.

For example, when using `ghq` to manage projects, plugin directories may
vary by onewrs of the plugins.

With this change, users can do something like below

``` lua
require("lazy").setup("plugins", {
  dev = {
    path = function(p)
      -- ghq
      local path, cnt = string.gsub(p.url, "^https://(.*)%.git$", "~/ghq/%1")
      if cnt == 1 then
        return path
      end

      -- fallback to default
      return "~/projects/" .. plugin.name
    end,
  },
})
```

README.md

@@ -320,7 +320,7 @@ return {
     filter = true,
   },
   dev = {
-    -- directory where you store your local plugin projects
+    ---@type string | fun(plugin: LazyPlugin): string directory where you store your local plugin projects
     path = "~/projects",
     ---@type string[] plugins that match these patterns will use your local versions instead of being fetched from GitHub
     patterns = {}, -- For example {"folke"}