chore: integrate cypress-axe for accessibility testing

[nnaydenow]

Note: These changes are intended for use within the monorepo only

These changes are integrating the cypress-axe plugin to enable accessibility testing within the UI5 webcomponents project. As part of the integration, a new CYPRESS_UI5_ACC environment has been added to control the execution of accessibility tests. When set to true, it allows generating an accessibility report otherwise, the tests are skipped.

Additionally, a new global function, ui5AccDescribe, has been introduced for test files. This function acts as a wrapper around the standard describe function, accepting the same parameters. Internally, it determines whether to run or skip tests based on the CYPRESS_UI5_ACC value. The wrapper also injects the necessary axe-core tool and includes the ui5CheckA11y command, which is used for accessibility validation.

Note: If CYPRESS_UI5_ACC is set, only accessibility tests will run

Furthermore, the ui5CheckA11y command has been added to execute axe on mounted content.