Allow `+` for deep customizations on `core:install`

__Background__: In several commands (`api3`, `api4`, `setting:set`), there are two kinds of options:

* _Control options_: Arguments like `--level`, `--out`, `-T`, `--user`, `--cwd`, `--hostname` influence the way that `cv` works. The naming / formatting / behavior of the options are primarily driven by `cv`.
* _Data model options_: Arguments like `+s field_1,field2` influence the data that `cv` passes through to `civicrm`.The naming / formatting / behavior are primarily driven by CiviCRM's model. The convention here is tuned for "adding data to a request-document". If you need to get really detailed, it allows JSON.

__Problem__: The `core:install` command has a mix of _control options_ and _data model options_. But the data-model options don't follow the `+` convention; they miss a bunch of edge-cases; and they don't allow JSON. The same is true of `core:check-req` and `core:uninstall`.

__Example__: Currently, you can use `cv path -m X=Y`, but there's no way to set a value for `$setup->model['paths']['civicrm.files']`.

__Idea__: In the same vein as `SettingArgParser` or `Api4ArgParser`, add a `SetupModelParser` to allow more inputs. Follow the same `+` conventions as the other commands.

Start by comparing the existing SettingArgParser and Api4ArgParser conventions, then trace argument handling for core:install, core:check-req, and core:uninstall. Done means these commands accept the same + data-model conventions, including JSON and nested setup-model values such as civicrm.files.

Written by the indexing model from the issue text.