createRegexQuery only transforms lowercase to uppercase and not the inverse

The problem

When building the regular expression for a facet query, createRegexQuery runs the following check, which takes any lowercase letters and adds an uppercase equivalent in a group to allow for case insensitivity. However this does not do the inverse. For example, the following transformation works as expected:

hello => [hH][eE][lL][lL][oO]

But the following don't do the same thing for uppercase to lowercase:

Input	Expectation	Actual
Hey	`[hH][eE][yY]`	`H[eE][yY]`
HeY	`[hH][eE][yY]`	`H[eE]Y`
HEY	`[hH][eE][yY]`	`HEY`

It feels like whatever is input here should be converted into upper and lower case to account for the lack of case sensitivity in a term query.

I'm happy to provide a PR to amend the createRegexQuery function if this is an issue that you'd be open to having fixed?

Environment

Searchkit version (or git revision) that exhibits the issue: 4.12.0
Last Searchkit version that did not exhibit the issue (if applicable): Unknown
Node.js version: 20.14.0
Typescript version: 5.4.5
Next.js version: 14.2.3
ElasticSearch/OpenSearch version: unrelated

Details

See above

Code To Reproduce Issue

Create an index with a single field

{
  "mappings": {
    "title": {
      "type": "text",
      "fields": {
        "keyword": {
          "type": "keyword",
          "ignore_above": 256
        }
      }
    }
  }
}

Add a document with the title of "Mario"
Add a RefinementList component looking at the field
Mario and mario return results
MaRiO and MARIO do not return results

searchkit / searchkit