Export timeunit.format() for VgTooltip

[light-and-salt]

Export timeunit.format(timeUnit, abbreviated) because tooltip needs to translate VL timeUnit to the time string specifier.

[domoritz]

Do we have to move the function? It's very inconsistent now.

[kanitw]

@domoritz what's inconsistent? (Ideally we shouldn't expose methods inside compile to the public?)

[domoritz]

What's inconsistent is that other methods stayed in time.ts (https://github.com/vega/vega-lite/blob/zqu/export-format/src/compile/time.ts). What's different about format? Is it the fact that it is made available to the outside? I don't have strong opinions just try to understand the reasoning.