Static type checkers can't recognize colormaps

Static type checkers like MyPy or Pylance can't recognize variables created dynamically with [`locals().update(cmap_d)`](https://github.com/matplotlib/cmocean/blob/59c35002c3aa5296b65d9646e52604c627441eb6/cmocean/cm.py#L84). Past just type checking, Pylance can't even recognize that colormaps exist, e.g., `cmocean.cm.balance` produces an [attribute access issue error](https://github.com/microsoft/pylance-release/blob/main/docs/diagnostics/reportAttributeAccessIssue.md) in Pylance.

Matplotlib colormaps have the same issue, which they solve by including a stub file [cm.pyi](https://github.com/matplotlib/matplotlib/blob/main/lib/matplotlib/cm.pyi) that defines the attributes and types.

If there's interest in include a cm.pyi file here, it is straightforward to generate one automatically based on the colormaps defined in `cmap_d` and I'd be happy to make a PR with the details.

Inspect cmocean/cm.py and the cmap_d definition to see how colormaps are created dynamically. Compare the referenced Matplotlib cm.pyi stub and determine how a generated stub would expose the colormap attributes and types; done means MyPy and Pylance recognize entries such as cmocean.cm.balance.

Written by the indexing model from the issue text.