ENH: #40231 does not allow writing multiple dataframes to a single sheet
#### The problem

The following code works well, both in 1.4.0 and 1.2.5:

```python
df = pd.DataFrame(["=1+7"])
with pd.ExcelWriter("test.xlsx",engine="openpyxl") as writer:
    df.to_excel(writer,sheet_name="Sheet2")
    pd.DataFrame(["=1+1"]).to_excel(writer,sheet_name="Sheet2",header=None,index=False,startcol=4,startrow=3)
```

however, the following code raises an error in 1.4.0, but works in 1.2.5 (it creates a new sheet and writes both dataframes to it):

```python
pd.DataFrame(["=1+1"]).to_excel("test.xlsx")
df = pd.DataFrame(["=1+7"])
with pd.ExcelWriter("test.xlsx",engine="openpyxl",mode="a") as writer:
    df.to_excel(writer,sheet_name="Sheet2")
    pd.DataFrame(["=1+1"]).to_excel(writer,sheet_name="Sheet2",header=None,index=False,startcol=4,startrow=3)
```

Also other options cannot replicate the old behaviour.

#### Describe the solution you'd like

An added option to `if_sheet_exists="write_to"` that justs selects the sheet and writes into it, without first deleting its contents. This also allows to append a dataframe within a sheet.

**EDIT:** in #40230 this was also suggested behaviour, but it was not implemented:
> Originally I considered adding a flag like `overwrite=True` to `pd.ExcelWriter` but I don't think this would be sufficient to cover all the potential behaviours, so would suggest an enum along the lines of the `if_exists` option in `df.to_sql`, with the following options:
> 
>     * new_sheet: Create a new sheet with a different name.
> 
>     * overwrite_sheet: Delete the contents of the sheet, then write to it.
> 
>     * overwrite_cells: Write directly to the named sheet without deleting the previous contents.
