Add singleton to indicate which shell version is requesting completion candidates

[rgoldberg]

Add singleton to indicate which shell version is requesting completion candidates.

Create a String? singleton named CompletionShell.requestingVersion that indicates which shell version is requesting completion candidates.

It will be set to the correct value while a Swift custom completion function is executing to offer completions for a word from a command line (e.g., while customCompletion from @Option(completion: .custom(customCompletion)) executes). Otherwise, it will be set to nil.

The requesting shell version is communicated to the Swift app via an environment variable named SAP_SHELL_VERSION, which is exported by each of the generated completion scripts.

Improve some nearby DocC.

Resolve #689

Replace this paragraph with a description of your changes and rationale. Provide links to an existing issue or external references/discussions, if appropriate.

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 read the Contribution Guidelines
• I've updated the documentation if necessary

[natecook1000]

@swift-ci Please test