Style compiler fails on COMPAT for nested var combined expressions

[diervo]

Description

Error generating CSS for compat for nested compounded variables

Steps to Reproduce

.a {
    box-shadow: var(--x, 0 0 2px var(--y, #fff));
}

Should produce:

function stylesheet(hostSelector, shadowSelector, nativeShadow) {
  return ".a" + shadowSelector + " {box-shadow: " + varResolver("--lwc-c-active-button-box-shadow","0 0 2px " + varResolver("--lwc-brand-accessible","#0070d2")) + ";}\n";
}

Instead, its missing the concatenation operator when Text and Expression are combined within a variable expression.