fix: no abbrs causing crash (#28)

[ndonfris]

Why?

Fixes and tests issues in src/utils/completions/startup-config.ts, when the user does not have any defined abbr, alias, or event. This patch should prevent piping empty outputs from any command above, to their trailing formatting commands.

- abbr | string split ' -- ' -n -m1 -f2 | string unescape
+ [ (abbr | count) -eq 0 ] || abbr | string split ' -- ' -n -m1 -f2 | string unescape

Other changes

• added relevant tests for test-data/completion-startup-config.test.ts & test-data/formatting.test.ts
• added relevant tests to yarn test-hook in package.json
• added husky pre-push hook

• added private test-data helper function, to spawn in a minimal fish profile. Relies on the following command:

  fish --no-config