[MNG-8344] Support multiple operators in variable expansion

[gnodet]

JIRA issue: MNG-8344

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The goal is to enhance the supported syntax for operators in variable expansion, so that we can use those as a ternary operator:

maven.user.config = ${env.MAVEN_XDG:+${env.XDG_CONFIG_HOME:-${user.home}/.config}/maven}:-${user.home}/.m2}