Support side border shorthands in CSSBorder

[jsfan3]

This adds support for side-specific CSS border shorthands in CSSBorder.

Context: the Initializr template CSS uses declarations such as:

SideCommand {
    border-bottom: 2px solid #cccccc;
}

This is accepted by the regular Designer/Maven CSS compiler, but the lightweight CSSThemeCompiler paths used by Initializr preview / agent validation eventually pass this declaration to CSSBorder, where it fails with:

Unsupported CSS property: border-bottom

The issue is that CSSBorder supported general border properties such as border-width, border-style, and border-color, but not side-specific shorthand properties like:

• border-top
• border-right
• border-bottom
• border-left

This PR adds support for those side-specific border shorthands, plus the corresponding side-specific longhands such as border-bottom-width, border-bottom-style, and border-bottom-color.

I also added a regression test covering:

SideCommand {
    border: none;
    border-bottom: 2px solid #cccccc;
}

Verification:

mvn -DunitTests -pl core-unittests -am -Dtest=CSSThemeCompilerTest -DfailIfNoTests=false test
mvn -DunitTests -pl core-unittests -am -Dtest=CSSBorderTest,BorderAndPlafTest -DfailIfNoTests=false test

Results:

• CSSThemeCompilerTest: 5 tests, 0 failures, 0 errors
• CSSBorderTest + BorderAndPlafTest: 18 tests, 0 failures, 0 errors

[shai-almog]

Thanks!