Add script to migrate a style from legacy functions and filters to expressions

[anandthakker]

We have a module for converting stop functions to expressions (src/style-spec/function/convert.js), so what we need is:

• A method to convert a legacy filter into an equivalent expression. This doesn't exist internally right now, because we use a specialized code path to handle legacy filters. Note that filters converted to expressions might be slightly less performant than their legacy counterparts because of the overhead necessary to represent the exact legacy type semantics in expressions.
• A function that takes in a style and traverses & converts all of its layer properties and filters.

cc @nickidlugash @mapbox/studio