fix: add missing a11y-base dependency to side-nav

[web-padawan]

Description

Trying to use @vaadin/side-nav package in a Vite app currently throws an error due to missing dependency:

✘ [ERROR] Could not resolve "@vaadin/a11y-base/src/disabled-mixin.js"

    node_modules/@vaadin/side-nav/src/vaadin-side-nav-item.js:8:30:
      8 │ import { DisabledMixin } from '@vaadin/a11y-base/src/disabled-mixin.js';
        ╵                               ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

✘ [ERROR] Could not resolve "@vaadin/a11y-base/src/styles/sr-only-styles.js"

    node_modules/@vaadin/side-nav/src/vaadin-side-nav-item.js:9:33:
      9 │ import { screenReaderOnly } from '@vaadin/a11y-base/src/styles/sr-only-styles.js';
        ╵                                  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Type of change

• Bugfix

[sonarcloud]

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0 Bugs
0 Vulnerabilities
0 Security Hotspots
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