fix: keep track of css injected to document (#11635) (#11673)

[caalador]

Stops css/fonts from being loaded multiple times to the document with embedded applications.

Fixes #11608

[vaadin-bot]

SonarQube analysis reported 6 issues

1. BootstrapHandler.java#L123: Define a constant instead of duplicating this literal "//<![CDATA[\n" 3 times.
2. BootstrapHandler.java#L124: Define a constant instead of duplicating this literal "//]]>" 3 times.
3. BootstrapHandler.java#L591: Use already-defined constant 'SCRIPT_TAG' instead of duplicating its value here.
4. BootstrapHandler.java#L590: Call "Optional#isPresent()" before accessing the value.
5. WebComponentBootstrapHandler.java#L80: Remove this call from a constructor to the overridable "setInitTheme" method.
6. WebComponentBootstrapHandler.java#L387: Remove the declaration of thrown exception 'java.io.UnsupportedEncodingException', as it cannot be thrown from method's body.