[SPIKE] Revisit vendoring, in favor of adding mason dependency

## Context
https://github.com/mapbox/vtshaver/pull/58
The above PR added a vendored nunicode to vtshaver build. There may be a simpler way to provide the necessary files to vtshaver, such as adding nunicode as a mason dependency.

Additionally, an environment variable `MBGL_USE_BUILTIN_ICU` was defined in the build to use a simpler version of [number-format](https://github.com/mapbox/mapbox-gl-native/blob/b8edc2399b9640498ccbbbb5b8f058c63d070933/platform/default/src/mbgl/i18n/number_format.cpp) that did not require ICU files. This might also be possible to provide as a linked dependency.

Start by reviewing PR #58 and the current vtshaver build setup, focusing on the vendored nunicode files and the MBGL_USE_BUILTIN_ICU environment variable. Compare whether mason can provide the required files and linked number-format dependency; done means the vendored files can be removed while the build still supports the simpler number-format path.

Written by the indexing model from the issue text.