Convert source code to 100% Swift

[wangela]

Convert source code from a mix of Objective-C and Swift to all Swift so the library can have a single target.

For reasons described in #473, the maintenance and contribution to this library are hindered by its mixed-language source and the fact that most of the source code is written in Objective-C. This feature request captures the work to translate the Objective-C code to Swift.

The work is already being done by @iHackSubhodip in several branches named feature/...-Swift:

• https://github.com/googlemaps/google-maps-ios-utils/tree/feature/Cluster-Swift
• https://github.com/googlemaps/google-maps-ios-utils/tree/feature/Geometry-Swift
• https://github.com/googlemaps/google-maps-ios-utils/tree/feature/HeatMap-Swift
• https://github.com/googlemaps/google-maps-ios-utils/tree/feature/QuadTree-Swift-main

The plan is to merge these into branch main-swift, add DocC documentation, and release as v7.0 of this library.