Add floating point parsers

[natecook1000]

Adds parsers for the standard library floating point types with endianness specifiers. Resolves #25.

Checklist

• I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've run Scripts/format.sh to correctly format my change
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

[natecook1000]

The API documentation is good for now – I'll follow up with DocC curation for these, the LEB parsers, and other APIs we add before the next release.

[stephentyrone]

This needs to be #if'd out on macOS/Intel.

#if !((os(macOS) || targetEnvironment(macCatalyst)) && arch(x86_64))
extension Float16 {
  // ...
}
#endif

[stephentyrone]

We may want to provide a Float(parsingFloat16:) as a workaround for macOS/Intel systems, but that can be a follow-on PR.

[stephentyrone]

As with Float16, if anything what I would provide here is a fallback bailable parser that tries to parse the Float80 into a Double (what to do for out-of-range values TBD).

[JaapWijnen]

@natecook1000 Are there plans to merge this? I'm currently maintaining a fork to keep this up to date with main. With some functionality for parsing certain container types on top (Array, InlineArray, SIMD, etc.). I'd love to contribute those if they sound interesting but I'm currently blocked by requiring the floating point parsers.

I'd love to push it to the finish line. If anything else is required, please let me know!