rule: Require ordering of default rule before other rule heads

[charlieegan3]

This is an idea for a new rule.

The example below would violate the rule, as the default keyword is after the other rule head.

package example

import rego.v1

allow if input.admin

default allow := false

This fix:

package example

import rego.v1

default allow := false

allow if input.admin

The rationale for this would be that as more rule heads are added, the default definition is pushed farther and farther down the file and this more likely to be missed.

This rule should only apply to rule heads in one source file, if the default is elsewhere, then that's ok.