feat: Removed dependency on Foundation

[bradhowes]

Description

Addresses issue #13

Removes Foundation dependency by replacing with Darwin (macOS) or Glibc (Linux), and providing a custom
String(format:...) constructor with one that is based on vsnprintf.

Originally used vasprintf per example code found in Apple doc, but Swift does not find this function in glibc.

How Has This Been Tested?

All existing tests pass, including testOutputPercentileDistributionPlainText which exercises the replacement function.

Minimal checklist:

• [ X] I have performed a self-review of my own code
• I have added DocC code-level documentation for any public interfaces exported by the package
• I have added unit and/or integration tests that prove my fix is effective or that my feature works

[hassila]

Ping @dimlio

[dimlio]

The changes looks good to me.

For some reason CI on macOS is failing (compiler crashed?!) but it build fine on my box (with Swift 6.0.2).