fix: handle backslash in font-icons when generating CSS

[web-padawan]

Description

Related to finding from vaadin/web-components#7976 (comment)

This fixes the output of CSS generator script to look like the one we had in the original repository.

Before

  html {
    --lumo-icons-align-center: '\\ea01';
    --lumo-icons-align-left: '\\ea02';
    --lumo-icons-align-right: '\\ea03';
    /* ... */
  }

After

  html {
    --lumo-icons-align-center: '\ea01';
    --lumo-icons-align-left: '\ea02';
    --lumo-icons-align-right: '\ea03';
    /* ... */
  }

Type of change

• Bugfix

Note

Can be tested by running the following npm script:

npm run build:generate-css -w @vaadin/react-components