feat: Rewrite customAttributeMixin to validation helper (#9916)

# Pull Request Template

## Description

This PR will replace the use of `customAttributeMixin` with
`shared/helpers/Validators` helper.

Fixes
https://linear.app/chatwoot/issue/CW-3446/rewrite-customattributemixin-mixin-to-a-composable

**Files updated**
1. widget/components/PreChat/Form.vue
2. dashboard/components/CustomAttribute.vue
3. dashboard/routes/dashboard/settings/attributes/EditAttribute.vue

## Type of change

- [x] New feature (non-breaking change which adds functionality)

## How Has This Been Tested?

Test the custom validation is working or not with the custom attributes.


## Checklist:

- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my code
- [x] I have commented on my code, particularly in hard-to-understand
areas
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [x] I have added tests that prove my fix is effective or that my
feature works
- [x] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged and published in downstream
modules

app/javascript/dashboard/mixins/customAttributeMixin.js

@@ -1,11 +0,0 @@
-export default {
-  methods: {
-    getRegexp(regexPatternValue) {
-      let lastSlash = regexPatternValue.lastIndexOf('/');
-      return new RegExp(
-        regexPatternValue.slice(1, lastSlash),
-        regexPatternValue.slice(lastSlash + 1)
-      );
-    },
-  },
-};