Add const to exact comparisons

[rosettaroberts-impact]

This improves postgres enum support by making as const not needed in conditions when creating conditions for columns with enum types.

This typescript feature requires typescript 5.0. This might be problematic because the peer dependency is "typescript": ">=4.1".

This is an example of the problem this PR is trying to fix.

Schema

declare module 'zapatos/schema' {
  export type enum_profile_avail_check_status = 'Avail' | 'Unavail' | 'Waiting';

  export namespace profile_avail_check {
    export type Table = 'profile_avail_check';
    export interface Whereable {
      /**
       * **profile_avail_check.status**
       * - `enum_profile_avail_check_status` in database
       * - `NOT NULL`, default: `'Waiting'::enum_profile_avail_check_status`
       */
      status?:
        | enum_profile_avail_check_status
        | db.Parameter<enum_profile_avail_check_status>
        | db.SQLFragment
        | db.ParentColumn
        | db.SQLFragment<
            any,
            | enum_profile_avail_check_status
            | db.Parameter<enum_profile_avail_check_status>
            | db.SQLFragment
            | db.ParentColumn
          >;
    }
  }
}

Before this change, the following code has a typescript error:

db.select('profile_avail_check', {
  status: db.conditions.ne('Waiting'),
});

The reason it has a typescript error is because the type of db.conditions.ne('Waiting') is inferred to be SQLFragment<boolean | null, string>, which is not a subtype of db.SQLFragment<any, enum_profile_avail_check_status>. A workaround is to instead do

db.select('profile_avail_check', {
  status: db.conditions.ne('Waiting' as const),
});

With the workaround, db.conditions.ne('Waiting') is inferred to be SQLFragment<boolean | null, 'Waiting'> which is a subtype of db.SQLFragment<any, enum_profile_avail_check_status>. However, it can be annoying to remember this workaround.

This PR removes the need for the workaround by telling typescript to infer the narrowest type possible for the parameter.

[jawj]

I think requiring TS 5 probably requires a new major version, so I'll keep this one on the back-burner for now.