Missing/misspelled variable name recommendations

This is the equivalent of PR #1776 but for variables instead of keywords. 

I've moved some of the reusable recommendation code from `namespace.py` to `utils/match.py` in a new `FuzzyMatcher` class. Trying to balance code duplication and complexity in this refactoring was tricky, so criticism/ideas are welcome.

There are a lot of special cases for variable naming, including accessing variable indexes, accessing extended variables, and accessing nested variables. I have only partially handled these special cases - recommendations for misspellings of the base variable name will generally work, but anything more complex will not work, and the recommendations will not include the index or extended variable syntax. 

I reworded some of the missing variable error messages, just to make them subjectively sound better. For example, `Non-existing variable '%s'` is now `Variable '%s' not found`. This breaks many tests throughout `atest`, but in this PR I have not fixed those tests yet. I'll fix them if this rewording/rephrasing is approved. 

For example recommendations, please see the new test cases I've added.
No need for `_` prefix here either. Could just reassign `name` or alternatively use

    msg = "Environment variable '%%{%s}' not found." % name
    self._raise_non_existing_variable('%%{%s}' % name, msg, env_vars=True)