Support format expression function about vegafusion

[jonmmease]

Looking at the generated Vega spec, there is this transform

        {
          "type": "formula",
          "expr": "!isValid(datum[\"bin_maxbins_20_doy\"]) || !isFinite(+datum[\"bin_maxbins_20_doy\"]) ? \"null\" : format(datum[\"bin_maxbins_20_doy\"], \"\") + \" – \" + format(datum[\"bin_maxbins_20_doy_end\"], \"\")",
          "as": "bin_maxbins_20_doy_range"
        },

The issue is that the format expression function isn't implemented yet. I have an old Rust implementation of the Vega format function that I developed a while back for another project. I'll see if I can port that over to VegaFusion. Although, it may also be worth adding a special case for format(value, "") that just transforms this into toString(value), as this will be easier to support across SQL backends.

Also cross reference #336 for better error messages when something isn't supported.