`cql2_like_to_es()` does not correctly convert

[kurtmckee]

Describe the bug

cql2_like_to_es() doesn't handle escaped backslashes, which leads to the following problems:

• '\\' (an escaped backslash) is not converted to '\'.
• '\\%' (an escaped backslash followed by a wildcard) is not converted to '\*'
• '\\\% (an escaped backslash followed by an escaped percent sign) is not converted to '\%'.
• '\\_' (an escaped backslash followed by a wildcard) is not converted to '\?'.
• '\\\_' (an escaped backslash followed by an escaped underscore) is not converted to '\_'.

In addition, CQL2 doesn't appear to define what should happen when a trailing escape character is encountered (like 'abc\') nor what to do with an invalid escape sequence (like '\1').

I recommend rejecting invalid/incomplete escape sequences.

To Reproduce

I'll be posting a PR with a break/fix momentarily for evaluation.

Expected behavior

I expected cql2_like_to_es() to understand escaped backslashes.

Screenshots
n/a

Desktop (please complete the following information):
n/a, this is a bug in the Python implementation

Smartphone (please complete the following information):
n/a

Additional context
n/a