More regex functions - find, substring, replace

[sc1f]

PR on top of #1596, but this one can be merged first to show all the changes - it's in a separate PR to make code review easier.

This PR adds more regex functions:

• find(string, pattern, output_vector): writes the start and end-index (0-indexed) of the first match in string of the first capturing group in pattern. Values can be read from the output vector as follows: var vec[2]; find("x", '(.*)', vec) ? vec[0] + vec[1] : null. Returns true if a match was found and values were written to the output vector, and false otherwise.
• substring(string, start_idx, length?): returns the substring of string from start_idx (0-indexed) with the specified length. If the length is not provided, returns the substring from start_idx to the end of the string, or null if the string is invalid or indices are invalid.
• replace(string, pattern, replacer): replaces the first match of pattern in string with replacer, and returns the replaced string or the original string if no replacements were made.
• replace_all(string, pattern, replacer): replaces all matches (non-overlapping) of pattern in string with replacer, and returns the replaced string or the original string if no replacements were made.

This PR also renames fullmatch from #1596 to match_all to remain consistent with the replace/replace_all API.

With this suite of Regex functions, one can easily clean and convert data from within Perspective - poorly formatted strings, dates that could not be parsed, monetary values that you want to convert to a float but cannot because of other string tokens, etc. can all be cleaned and properly converted from within Perspective's expression API.

[texodus]

Looks good! Thanks for the PR!