Capitalize global navigation constants

[aaronfranke]

While working on PR #89409 I couldn't help but commit some scope creep and capitalize these constants, but I don't want to overload that PR with so much so I moved this to a separate PR.

This PR:

• Capitalizes all constants in servers/navigation/navigation_globals.h
• Adds an underscore after NAV for readability. (the prefix is useful because in a few places it's using namespace'd).
• Renames the 2D ones from NAVMESH to NAV_POLYGON.
• Replace {} initialization with = initialization for simplicity.
• Moves the range hint into where it's used in the server cpp files (each was only used once).
  • This also eradicates a usage of auto 🥳
• Fix a few places in docs and error prints where NavigationMesh was used instead of NavigationPolygon.

[smix8]

Fine with the upper() renames but please revert most renames from navmesh to navpolygon. Those that are doc fixes and have the actual NavigationPolygon resource are fine but those for properties and general documentation should be just navmesh everyhwere. We try to have navmesh / navigation mesh for both 2d and 3d wherever possible to make code and doc sharing easier and clearer as polygons mean the actual polygons inside the navmesh. That there is a name collision with the 2d NavigationPolygon resources is unfortune but it is what it is.

[Repiteo]

Thanks!