Add opt-in traits for optional vector tile formats

[trasch]

Adds fine-grained SwiftPM traits to mvt-tools so consumers only build the formats they need. MVT and GeoJSON stay always-on (core); MLT, CSV, GeoPackage, GPX, FIT, and Shapefile become opt-in via Enable traits.

Highlights:

• MVTTools target now gates optional GISTools dependencies behind trait conditions
• C++ interop (.interoperabilityMode(.Cxx)) is only enabled when EnableMLT is on, so a default build requires no C++ interoperability
• Coder files and their tests are wrapped in #if EnableX
• CI (swift test) and the mvt CLI release build/install script now use --enable-all-traits

Breaking (major release): previously all formats were compiled unconditionally.