Move the geofield formatter into a cesium_geofield submodule

Problem/Motivation

The current module requires drupal/geofield, because its headline feature is the Cesium Globe geofield formatter. Providing the CesiumJS library and the Ion settings does not need geofield, but a site that only wants those gets geofield anyway.

That matters beyond tidiness. A separate module for the OpenLayers 2D to 3D bridge will depend on this one for library provision, and a site using it with WMS layers would pull in a field type it never uses.

Proposed resolution

Split the module in two.

Base cesium keeps what has no field dependency: the CesiumJS library declaration, the Ion token settings form and its configuration, and CESIUM_BASE_URL.

New cesium_geofield submodule takes the geofield formatter and declares drupal/geofield itself.

Existing sites enable both and behave as before.

Remaining tasks

• Move the formatter, its templates and its tests into the submodule.
• Remove drupal/geofield from the base module's composer.json and info.yml.
• Decide whether an update hook is needed to enable cesium_geofield on sites already using the formatter, or whether the release note is enough at this stage.
• Update the README so the two modules and what each provides are clear.

User interface changes

None for a site that enables both modules. The formatter keeps its name and its settings.

API changes

The base module no longer declares drupal/geofield. Anything depending on cesium for the formatter should depend on cesium_geofield instead.

Data model changes

None. The formatter's stored settings are unchanged; only the module that provides it moves.