Add regex built-ins

[mathcat]

eg. first match, all matches, return groups, ...

[lyxal]

Regex features (stolen from python's re library):

• Search => return first index of pattern match
• Match => return the portion of the string that pattern matches
• Matches at all => return a boolean whether a match is present
• Matches Fully => return whether the entire string matches the pattern
• Split => Split a string on a pattern
• Find All => find all non-overlapping matches of pattern in string
• Find All overlapping => Find all but more painful to implement
• Substitute => Replace instances of pattern with string/function taking match as input
• Escape => Take a string and escape it
• Groups => Return a list of all groups of a match
• Span => Return the start and end position of a match

Strikethrough represents implemented.