Split `Analysis.Group.Normed.Basic` into three files:
- `Analysis.Group.Normed.Basic` contains the definitions and the basic results that just use the metric space structure
- `Analysis.Group.Normed.Continuity` contains the more advanced results especially the ones that uses the continuity of the norm 
- `Analysis.Group.Normed.Subgroup` contains the results about subgroups of normed groups.

Question. Is there a better name than `Analysis.Group.Normed.Continuity`?

Splitting it out in #21554 was motivated by avoiding imports of ENNReal.
This motivation has become moot now:
- with #22562, just the basic properties of ENNReal do not add that many
  further imports
- after #21670, Normed/Group/Basic.lean will import ENNReal anyway.

Adding to this, the split into Basic and Continuity was never really clear
in terms of imports: Basic.lean should be split further, but this does not seem
like a good splitting boundary.

While at it, replace one 'open' that has been likely mis-ported by 'open scoped'.

Splitting it out in #21554 was motivated by avoiding imports of ENNReal.
This motivation has become moot now:
- with #22562, just the basic properties of ENNReal do not add that many
  further imports
- after #21670, Normed/Group/Basic.lean will import ENNReal anyway.

Adding to this, the split into Basic and Continuity was never really clear
in terms of imports: Basic.lean should be split further, but this does not seem
like a good splitting boundary.

While at it, replace one 'open' that has been likely mis-ported by 'open scoped'.