Fixes syl20bnr#16357

The change expands the matching regex to only highlight lines where the debugger
is the first word on a line.

The intended behavior is to highlight lines that are calling a ruby debugger.
The purpose is to warn developers that might push code to production
with a debugger enabled. The change expands the matching regex to only highlight
lines where the debugger is the first word on a line.

However, there are cases in which the regex is highlighting lines as a
false-positive. I first noticed this in my `Gemfile` when it was highlighting the
line below:

```ruby
gem 'pry-byebug'
```

Other cases include: commented lines and defined function names that match a known
debugger.

See below for cases in which the line should and should not be highlighted.

```ruby

class Test
  # This line below should not be highlighted
  def byebug
    byebug # this line should be highlighted
    # byebug This comment shouldn't be highlighted

    binding.irb # this line should be highlighted
    # binding.irb this line should not be highlighted

    binding.pry # this line should be highlighted
    # binding.pry this line should not be highlighted
  end
end
```

Fixes #16357

The change expands the matching regex to only highlight lines where the debugger
is the first word on a line.

The intended behavior is to highlight lines that are calling a ruby debugger.
The purpose is to warn developers that might push code to production
with a debugger enabled. The change expands the matching regex to only highlight
lines where the debugger is the first word on a line.

However, there are cases in which the regex is highlighting lines as a
false-positive. I first noticed this in my `Gemfile` when it was highlighting the
line below:

```ruby
gem 'pry-byebug'
```

Other cases include: commented lines and defined function names that match a known
debugger.

See below for cases in which the line should and should not be highlighted.

```ruby

class Test
  # This line below should not be highlighted
  def byebug
    byebug # this line should be highlighted
    # byebug This comment shouldn't be highlighted

    binding.irb # this line should be highlighted
    # binding.irb this line should not be highlighted

    binding.pry # this line should be highlighted
    # binding.pry this line should not be highlighted
  end
end
```