Server mode for Jupyterlab PerspectiveWidget

[texodus]

• [@sc1f] Adds server kwarg to PerspectiveWidget() Jupyterlab widget. When True, the browser <perspective-viewer> will use a python, server-side Table() virtually, rather than creating a WebAssembly Table() and cloning the contents. This mode is great for extra-large datasets, but comes at the cost of client interactive performance, and server memory/CPU.
• Fixes 2 discrepancies with the Javascript API:
  • Fixes missing offset (called psp_offset in wasm) for correcting column_only end_col values.
  • Adds id kwarg for to_columns(), which is equivalent to __index__ for 0-sided contexts, and __row_path__ otherwise, and is necessary for GUI features like persistent selection and editing.
• Fixes a longstanding bug in perspective-jupyterlab which caused Create New Output derived viewers to be mis-sized.
• Fixes perspective-viewer-datagrid to not swallow scroll events when fully-scrolled, for convenient notebook scrolling.
• Switches perspective-jupyterlab to material-dense theme.

[sc1f]

Tested locally - looks good! Proper styling for the additional output view is nice.