use libnu mason dependency instead vendored src

[axrj]

• Fixes [SPIKE] Revisit vendoring, in favor of adding mason dependency #60
• Adds libnu 1.8 as mason dependency and updates build accordingly
• Removes vendored source code of libnu

make test fails with missing symbol called error on mac with both main and the PR branch equally. tests pass on CI though.

Todo:

• Test with api-vectortiles
• Maybe check test failure on mac.

[sofiapag]

TBD - from slack convo
What does make test do that is different from Travis builds?
Which missing symbol is missing on main? It could be a symbol which is undefined but not used in shaving, in which case it would be ok.

[axrj]

ah.
symbol not found in flat namespace '__ZTVN4mbgl12LayerFactoryE'
basically mbgl.LayerFactory
I don't think this is used anywhere but a reference is being compiled on darwin platform. Not sure where yet.