Port brew help (without arguments) to Bash

[MikeMcQuaid]

This provides a decent speedup:

$ hyperfine 'git checkout master; brew help' 'git checkout help_bash; brew help'
Benchmark 1: git checkout master; brew help
  Time (mean ± σ):     506.4 ms ±  50.9 ms    [User: 223.7 ms, System: 99.9 ms]
  Range (min … max):   454.6 ms … 634.1 ms    10 runs

Benchmark 2: git checkout help_bash; brew help
  Time (mean ± σ):     109.5 ms ±  57.1 ms    [User: 1

and compares favourably to pip3 help:

$ hyperfine 'brew help' 'pip3 help'
Benchmark 1: brew help
  Time (mean ± σ):      72.9 ms ±  15.9 ms    [User: 4.9 ms, System: 6.3 ms]
  Range (min … max):    53.6 ms … 126.6 ms    31 runs

Benchmark 2: pip3 help
  Time (mean ± σ):     171.5 ms ±   6.1 ms    [User: 131.6 ms, System: 24.7 ms]
  Range (min … max):   164.2 ms … 189.3 ms    15 runs

Summary
  brew help ran
    2.35 ± 0.52 times faster than pip3 help