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

[sofiapag]

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 that did not require ICU files. This might also be possible to provide as a linked dependency.