Evaluation order of module-level initializers

[jclark]

Spec should define the order in which module-level initializer expressions are evaluated. This makes a difference since they may have side-effects. The goal should be to move things around as little as necessary to ensure that the variables that an initializer depends on are initialized before the initializer is evaluated.

[sameerajayasoma]

Relates to #101

[jclark]

See this use case: https://github.com/ballerina-platform/ballerina-lang/issues/30042

[jclark]

Evaluation order must satisfy the constraint that variables (or more module-level identifiers) cannot be referenced before they are initialized. If there's no possible ordering satisfying the constraint, then there's a cycle and that's a compile-time error.