fix(cli): show warning when a user supplies an unknown subcommand

[jamesgeorge007]

• I have read the contribution documentation for this project.
• I agree to follow the code of conduct that this project follows, as appropriate.
• The changes are appropriately documented (if applicable).
• The changes have sufficient test coverage (if applicable).
• The testsuite passes successfully on my local machine (if applicable).

Summarize your changes:
closes #1300

[codecov]

Codecov Report

Merging #1301 into master will decrease coverage by 0.3%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1301      +/-   ##
==========================================
- Coverage   68.43%   68.13%   -0.31%     
==========================================
  Files          67       67              
  Lines        1958     1958              
  Branches      378      378              
==========================================
- Hits         1340     1334       -6     
- Misses        444      451       +7     
+ Partials      174      173       -1

Impacted Files	Coverage Δ
packages/maker/flatpak/src/MakerFlatpak.ts	72.72% <0%> (-9.1%)	⬇️
packages/maker/appx/src/MakerAppX.ts	45.61% <0%> (-7.02%)	⬇️
packages/utils/async-ora/src/ora.ts	56.66% <0%> (-3.34%)	⬇️
packages/api/core/src/util/rebuild.ts	93.33% <0%> (+6.66%)	⬆️

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[malept]

Thanks, but this does not seem to be working with existing commands. Example:

$ electron-forge lint
✔ Checking your system
Usage: electron-forge [options] [command] <command>

Options:
  -V, --version  output the version number
  --verbose      Enables verbose mode
  -h, --help     output usage information

Commands:
  init           Initialize a new Electron application
  import         Attempts to navigate you through the process of importing an existing project to
                 "electron-forge"
  lint           Lints the current Electron application
  package        Package the current Electron application
  make           Generate distributables for the current Electron application
  start          Start the current Electron application
  publish        Publish the current Electron application to GitHub
  install        Install an Electron application from GitHub
  help [cmd]     display help for [cmd]
 Unknown command lint

✔ Linting Application