[ENHANCEMENT] Support alternative variable syntax for set suite/test/global variable

[d-biehl]

My Setup
    Run Tests   ${EMPTY}   core/test_suite_dir_with_init_file
    ${suite} =   Get Test Suite   Test Suite Dir With Init File
    ${subsuite_with_init} =   Get Test Suite   Sub Suite With Init File
    ${subsuite_without_init} =   Get Test Suite   Sub Suite Without Init File
    Set Suite Variable   $SUITE
    Set Suite Variable   $SUBSUITE_WITH_INIT
    Set Suite Variable   $SUBSUITE_WITHOUT_INIT

find references and so on

[d-biehl]

same for Get Variable Value, Variable Should Exist and Variable Should Not Exist

see here:

https://robotframework.org/robotframework/latest/libraries/BuiltIn.html#Using%20variables%20with%20keywords%20creating%20or%20accessing%20variables