Don't allow setting a global if it was declared with a computable expression

[aljazerzen]

Schema:

module default {
    global test := 'hello';
}

> set global test := 'world'; <--- this should error out, but it does not
OK

> select global test;
{'hello'}    <--- because this is confusing

[msullivan]

We'll also want to test that it fails when trying to set it directly from a client, also. That might be either more or less gnarly.

See test_edgeql_globals_composite for a test where we set globals using the protocol