bug(cli): command completion generation requires `<INPUT>` parameter

[JarvisCraft]

Description

Generating Clippy's tab-completions support requires the dummu <INPUT> parameter to be present.

Steps to reproduce

1. Run

$ jrsonnet generate bash

bash can be replaced with any supported shell.

Expected behaviour

Tab-completion code is generated for the provided shell.

Observed behaviour

error: The following required arguments were not provided:
    <INPUT>

USAGE:
    jrsonnet [OPTIONS] <INPUT> [SUBCOMMAND]

For more information try --help

Tab-completion code is geneated only in presence of (any) <INPUT> parammeter value.

[CertainLach]

There was no default subcommand support in clap, so it wasn't possible to implement both

jrsonnet generate
jrsonnet file.jsonnet

clap-rs/clap#975

Now, as it is implemented, there would be two subcommands:

jrsonnet generate bash
jrsonnet compile file.jsonnet

And if subcommand is not specified, it should default to jrsonnet compile

[CertainLach]

Unfortunately, it seems to be still not possible to implement correctly with current clap api

[CertainLach]

Actually, found workaround.