Fix/337 optional label radio checkbox

[marek7gol]

Summary

Fixes #337 - Webform item labelling not working for radio/checkbox groups.

This PR replaces the CSS-based approach for displaying "(optional)" labels with PHP preprocess functions, which provide more reliable and consistent behaviour across all form element types.

Problem

The existing CSS-based solution using --form-item-optional-pseudocontent had several issues:

1. Radio/checkbox groups: The "(optional)" text was incorrectly displayed on individual radio/checkbox control labels when selected, rather than on the fieldset legend.
2. Required parent elements: When a radio or checkbox was part of a required fieldset (using Webform's "Radios" or "Checkboxes" parent elements), the individual control labels still incorrectly displayed "(optional)" when selected.
3. Translation: The CSS-based approach using pseudo-content cannot be translated for multilingual sites (Irish, Welsh, etc.).

Solution

This PR implements two PHP preprocess functions that handle the "(optional)" label logic:

1. localgov_microsites_base_preprocess_fieldset()

Appends "(optional)" to the fieldset legend for supported element types: radios, checkboxes, webform_uk_address, webform_address. Only applies when the fieldset (parent element) is not required and has a legend title.

2. localgov_microsites_base_preprocess_form_element()

Appends "(optional)" to standalone (not being a part of a fieldset/parent element) form element labels. Supported types: checkbox, radio, textfield, textarea, email, tel, number, url, date, datetime, select. Detects if a radio/checkbox is part of a fieldset/parent element (by checking #array_parents depth) and skips individual labels in that case, as the legend already displays the "(optional)" text.

Changes

1. Added two preprocess functions to localgov_microsites_base.theme file.
2. Removed --form-item-required-pseudocontent and --form-item-optional-pseudocontent CSS variables from css/base/variables.css file.
3. Removed references to these CSS variables from css/components/forms.css file.

[marek7gol]

Hi @danchamp, @markconroy, @msayoung 👋
This PR is ready for review. It addresses the issues raised in #337 regarding the "(optional)" labelling for radio/checkbox groups. I'd appreciate your feedback when you have a moment. Thanks!

[dedavidson]

@markconroy would you able to take a look at this?