parser,checker: add top level comptime match support

[kbkpbot]

This PR enable top level $match.

Add @token support in $if and $match.
So we can write code such as :

$match $OS {
    'linux' {
        import mymodule_linux
    }
    'windows' {
        import mymodule_windows
    }
    'macos' {
        import mymodule_macos
    }
    $else {
        import mymodule_default
    }
}

Top level comptime $if or $match only support @token @MOD @OS @CCOMPILER @BACKEND or @PLATFORM right now.

I am not sure other @token is useful or not at top level: @VROOT, @VMODROOT, @VEXEROOT, @FN, @METHOD, @STRUCT,
@VEXE, @FILE, @DIR, @LINE, @COLUMN, @VHASH, @VCURRENTHASH, @VMOD_FILE,
@VMODHASH, @FILE_LINE, @LOCATION, @BUILD_DATE, @BUILD_TIME, @BUILD_TIMESTAMP

[huly-for-github]

Connected to Huly®: V_0.6-24612

[spytheman]

Excellent work.

[spytheman]

Top level comptime $if or $match only support @MOD, @OS, @CCOMPILER, @BACKEND or @PLATFORM right now.

I think that is fine. The rest have values, that can not be compared easily against known ones (except perhaps the build ones, but I am not sure, if it is a good idea to do compile time logic based on them anyway), without also making builds a lot more brittle.