Add /xxx/ regex syntax into KQL?

[markharwood]

With the advent of the new wildcard field we can expect to see more regex searches, because of the acceleration this field provides.

Currently KQL does not provide a way for users to type regular expressions.
I opened #66735 to consider a tool to help people debug/test regexes but this issue is about expanding the syntax of KQL to allow in-line expressions the same way Lucene's query string syntax does e.g.

/ the quick brown f.*/

The KQL input should ideally render to a RegExp query rather than a generic query_string expression. One of the flags we hope to expose in the RegExp query is a case sensitivity matching option. In most regex syntaxes this is conventionally enabled by adding an i (for insensitive) on the end of the expression e.g.

/foo/i

This is an example of a setting that is unlikely to be exposed in query_string settings so rendering RegExp objects would be better. However, unlike query_string the regexp query lacks the multi-field support that users might expect.

[timroes]

Duplicate of #46855