feat: add `%splitstrregex` builtin function

[The-Lum]

Hi all,

From this request:

• https://forum.plantuml.net/18827/%25splitstr-please-add-regex-support-as-second-argument

Here is a PR, with:

• [x] adding test for SplitStr
• [x] adding new SplitStrRegex builtin function
• [x] adding test for SplitStrRegex

To allow: %splitstrregex("FooBar", "(?=[A-Z])") -> ["Foo","Bar"]

For the namming...:

• Perhaps on the future... to rename to simply %split... And:
• See to keep or not the old SplitStr and %splitstr...

Ack. & Ref.:

• https://forum.plantuml.net/15374/delimited-string-split-into-an-array
• https://forum.plantuml.net/18827/%25splitstr-please-add-regex-support-as-second-argument
• https://www.javacodeexamples.com/java-stringtokenizer-using-regex-pattern/3570

Regards, Th.