Puppet highlighter: Fix over-greedy regexp detection

[nfagerlund]

The regex token was allowing /regexps/ to start after the word node or after
any non-word character. This ends up being too greedy! For example, take this
common code, using variable interpolation in a string that represents a file
path:

file { "${master_config_dir}/.ssh":
  ensure => file,
  owner  => 'jenkins',
  group  => 'jenkins',
  mode   => '0600',
}

The highlighter will start a regexp at /.ssh, in the middle of the string, and
continue for however many lines it takes to reach another slash.

So instead, let's be more conservative about where we might find a regexp. I
suggest:

• After node.
• After one of the following characters: ~=([{,
  • This catches usage in variable assignment, the =~/!~ operators,
    function calls, case statement and selector blocks, arrays, and data type
    objects that accept parameters (Pattern[/.../], etc.).
• After => (and the lesser-used +>) for hashes and resource attributes.
• At the start of a line, for, e.g., the LHS of the in operator. (I'm not 100%
  sure we should cover this case for a simple highlighter like Prism, and it's
  the one I'd most expect to cause problems later, but... I think it's ok...)

This commit appears to fix the worst of the mid-string blowouts.

[nfagerlund]

Pinging @Golmote b/c you submitted the original highlighter component.

[Golmote]

Hi @nfagerlund!
Your PR looks ok to me, and it does not break any existing tests. Merging now. ;)

[nfagerlund]

yaaaay, thanks!