This PR aims to create a Symbols column field in the XY-scatter plot allowing users to specify a mapping between specific row values and symbols. Currently this only supports the basic d3 filled symbols, but in the future we may extend this to allow custom symbols.

CHANGES:

• Rename default_config plugin field to plugin_attributes and extend its functionality. It now handles general purpose plugin attributes, which determine which attribute and style fields show up in the sidebar.
• Add plugin_attributes to the d3fc plugin and update its restore and save methods to communicate with the viewer.
• Create a "symbols" key-value mapping section in the column settings sidebar's styles tab.
• Rearrange some files in the column settings sidebar for clarity.
• Allow CustomEvents to be triggered from a model, and use this to send configs to/from plugins.
• Always show the Attributes tab; add the "Type" section, and disabled the expression editor for non-expression columns.
• Address view/session and FFI concerns in fix gradient values #2384
• Add tests

TODO:

• Style the symbol attribute section + general UX improvements
• Check edge cases
• Ensure the plugin maps symbols based on the symbol field (this is currently working for legends but not the chart itself)
• Override certain values based on the passed symbols config
• Add and update tests!

KNOWN ISSUES:

• Incompatible with color-by
• Incompatible with filter (where)
• Incompatible with split-by
• Incorrect behavior with group-by