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GH-37252: [MATLAB] Add `arrow.type.DateUnit` enumeration class (#37280)

### Rationale for this change

In support of adding  `arrow.type.Date32Type` and `arrow.type.Date64Type`, this pull request adds a `arrow.type.DateUnit` enumeration class mirroring the `arrow.type.TimeUnit` enumeration class.

`arrow.type.DateUnit` will have two values: `Day` and `Millisecond`.

### What changes are included in this PR?

1. Added a new enumeration class `arrow.type.DateUnit` which mirrors the `arrow.type.TimeUnit` enumeration class.

**Example**:

```matlab
>> day = arrow.type.DateUnit.Day

day = 

  DateUnit enumeration

    Day

>> millisecond = arrow.type.DateUnit.Millisecond

millisecond = 

  DateUnit enumeration

    Millisecond
```

### Are these changes tested?

Yes.

1. Added a new test class `tDateUnit` for `arrow.type.DateUnit` tests.
2. Updated code for `tTimeUnit` for consistency with `tDateUnit`.

### Are there any user-facing changes?

Yes.

1. There is now a new user-facing `arrow.type.DateUnit` enumeration class.

### Future Directions

1. Add an `arrow.internal.validate.temporal.dateUnit` helper function that mirrors `arrow.internal.validate.temporal.timeUnit`.
2. #37229
3. #37230
* Closes: #37252

Lead-authored-by: Kevin Gurney <kgurney@mathworks.com>
Co-authored-by: Sarah Gilmore <sgilmore@mathworks.com>
Signed-off-by: Kevin Gurney <kgurney@mathworks.com>