[rules] [strings] Add support for concat_split rule and splitted rules in prefix and suffix version

[vinisilvag]

This PR adds the concat_csplit rule from the Strings theory.
Rule reference can be found in the cvc5 documentation and in the AletheLF specification.

Furthermore, it:

• Splits the rule into two versions: application over prefixes and over suffixes;
• Makes the necessary modifications to allow other rules to be split in this way;
• Standardizes variable names to how they appear in AletheLF documentation;
• Fixes a bug in the function that creates Skolems in the concat_csplit rule.

[bpandreotti]

Just a few typos