Overhaul of the externally-facing API

[carnesen]

This PR is a fairly significant non-backwards-compatible overhaul to the externally-facing API of this package. See https://github.com/alwaysai/cli/pull/95 see see the ramifications of this change on the alwaysai CLI. This PR also increases unit test coverage of this library to ~98%.

Top-level API naming convention: Use PascalCase for factories instead of camelCased-"create" words, e.g. createJsonInput --> CliJsonInput. Consistently include the identifier "Cli" for better grep-ability, e.g. UsageError => CliUsageError. These changes are all pretty straightforward to incorporate into a consumer such as the alwaysai CLI via global search and replace.

Change property names of the CliInput type: args --> positionalInput, options --> namedInputs, and escaped --> escapedInput. These new names are more descriptive and grep-able.

Replace createCli + runAndExit by:

• CliArgvInterface: Takes a root command (leaf or branch) and returns a function of type (...argv: string[]) => Promise<any>. The returned argvInterface implements the core logic of selecting a command, transforming the argv strings into typed input values, and feeding them into the command's action function. Unlike the old createCli, the argvInterface consistently throws JavaScript Error objects. On success, argvInterface returns the result of the action function like old createCli result did. The argvInterface can be used for unit testing or for example to export a pure-JavaScript interface to CLI's command logic. E.g. to invoke the alwaysai CLI, I could invoke it from the command line as aai or I could do const { aai } = require('alwaysai').
• runCliAndExit: Similar to CliArgvInterface, this is a function that takes as a root command as its first argument. It passes the root command into CliArgvInterface and then invokes the argvInterface with process.argv as input. If argvInterface succeeds, runCliAndExit calls process.exit(0). If argvInterface throws, the exception is caught and handled. If the exception is a CLI_USAGE_ERROR error, runCliAndExit console.errors a usage string for the command then process.exit(1)s. If the exception is a CLI_TERSE_ERROR error, runCliAndExit console.errors the message of the error. Otherwise the exception is console.errored in its entirety. This logic is similar to the error handling of the old createCli.

Both CliArgvInterface and runCliAndExit take an optional enhancer argument inspired by the enhancer feature of Redux. The enhancer signature is (argvInterface) => argvInterface. Enhancers allow you to wrap the argvInterface to do stuff before or after it runs. See the alwaysai CLI enhancer for an example that logs/tracks command invocations and exceptions.

As a new feature I rolled into this change, in runCliAndExit, if the argvInterface throws an error with a numeric "code" property, it process.exits with that code.

[carnesen]

Verbally approved by @ryanmalone