check for chart type

[shosaco]

When a user forgets to add a type to hchart, a non-informative error is thrown:

library(highcharter)
hchart(mtcars, hcaes(x = mpg, y = cyl))
#> Error: Assigned data `type` must be compatible with existing data.
#> x Existing data has 1 row.
#> x Assigned data has 2 rows.
#> i Row updates require a list value. Do you need `list()` or `as.list()`?
#> Warning in if (type == "heatmap") {: Condition has length > 1 and only the first element will be used.
#> 2: In if (type %in% c("bubble", "scatter")) { : Condition has length > 1 and only the first element will be used.

This PR enables the following behaviour:

library(highcharter)
hchart(mtcars, hcaes(x = mpg, y = cyl))
#> Error: Chart type must be provided.

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