Support initial selections in geo charts

[jonmmease]

Closes https://github.com/vega/vl-convert/issues/136

Sets the special geo_interval_init_tick signal value to support initial selection in geo charts.

See:

• https://github.com/vega/vega/issues/3481
• https://github.com/vega/vega-lite/pull/6953/files/50ab21f2ccbbb69d9d2e8398618ba0a68a7495ae#diff-dc401ea80fa16cb921f07dab3296ed956e3657f51fa2f673646856f129302823R19