Add a API doc examples for split operation in lang.regexp with used keywords

[SasinduDilshara]

We need to add a sample like following the lang.regexp API docs.

string:RegExp r = re `\|`;
r.split("abc|cde|efg") ⇒ ["abc","cde","efg"]

[SasinduDilshara]

This issue is based on the requirement from a external user.

[MaryamZi]

If this is an API doc example, the change needs to happen in ballerina-lang. Let's move the issue if so.