Prevent highlighting portions of variables that match built-in syntax.

[knoepfel]

In the following Jsonnet snippet, the skip_if parameter contains an underscore, which is recognized as a word boundary in jsonnet-mode. In this case, the if component of the parameter is highlighted whereas the skip_ substring is not.

local gather_from(arr, field, skip_if="") =
  std.foldl(function(res, config) append(res, config[field], skip_if), arr, {});

{
  sources_from(arr): gather_from(arr, "sources", skip_if="source_channel"),
  publishers_from(arr): gather_from(arr, "publishers"),
  transforms_from(arr): gather_from(arr, "transforms"),
}

This PR adjusts the regular expressions according to the definition of a "symbol" instead of a "word", as defined here.

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project (see https://github.com/bbatsov/emacs-lisp-style-guide).
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.

[tminor]

LGTM, thanks!