to_json should make separators configurable (similar to json.dump)
#### Code Sample, a copy-pastable example if possible

```python
import numpy as np
import pandas as pd

print(pd.DataFrame([1]).to_json(indent=4))
```
outputs
```
{
    "0":{
        "0":1
    }
}
```
#### Problem description

Any JSON prettyfier (including bare VS Code) wants to replace all `:` by `: `. So output of `to_json` should behave as `json.dump` does, at least when `indent is not None` (in which case the user clearly does not care about file size):

> If specified, separators should be an (item_separator, key_separator) tuple. 
> **The default is `(', ', ': ')` if indent is None and `(',', ': ')` otherwise.** 
> To get the most compact JSON representation, you should specify (',', ':') to eliminate whitespace.

https://docs.python.org/3/library/json.html#:~:text=To%20get%20the%20most%20compact%20JSON%20representation

#### Expected Output
```
{
    "0": {
        "0": 1
    }
}
```
#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.8.2.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.12.14-lp151.28.32-default
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.0.3
numpy            : 1.18.2
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 20.0.2
setuptools       : 46.1.1
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : 1.2.8
lxml.etree       : 4.5.0
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : None
IPython          : None
pandas_datareader: None
bs4              : 4.8.0
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.5.0
matplotlib       : 3.2.1
numexpr          : 2.7.1
odfpy            : None
openpyxl         : 3.0.3
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : None
pyxlsb           : None
s3fs             : None
scipy            : 1.4.1
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : 1.2.0
xlwt             : None
xlsxwriter       : 1.2.8
numba            : None
</details>
