Replace `sub` with `delete_prefix`/`delete_suffix`

Ruby 2.5 introduced `delete_prefix` and `delete_suffix`.

Those methods are helpful when serializing formula and array formula
values, that are supposed to start and end with given prefixes

Also moves formula prefix to constants so they can be used by both
`Cell` and `CellSerializer` classes

Formula:
```
Ruby version: 3.2.2
Comparison:
       delete_prefix:  8759353.5 i/s
                 sub:  2607022.4 i/s - 3.36x  (± 0.00) slower

Comparison:
       delete_prefix:         40 allocated
                 sub:        160 allocated - 4.00x more

```

Array Formula:
```
Ruby version: 3.2.2
Comparison:
     delete_prefixes:  4798837.8 i/s
             sub_sub:   937072.1 i/s - 5.12x  (± 0.00) slower

Comparison:
     delete_prefixes:        120 allocated
             sub_sub:        488 allocated - 4.07x more
```

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