inst.to_data_frame() should allow to export index as datetime64

here is an example of script I just had to write for a collaborator:

from pathlib import Path
import pandas as pd
import mne

sample_dir = Path(mne.datasets.sample.data_path())
sample_fname = sample_dir / 'MEG' / 'sample' / 'sample_audvis_raw.fif'

raw = mne.io.read_raw_fif(sample_fname, preload=True)
raw.crop(tmax=10)

df = raw.to_data_frame()
df = df.set_index("time")

index =  pd.date_range(start=raw.info['meas_date'],
                       periods=len(df) + raw.first_samp,
                       freq=f'{1e3 / raw.info["sfreq"]:0.6f}ms')
df.index = index[raw.first_samp:]

what I have in mind is that we can do

raw.to_data_frame(time_format='date')

to get the time as datetime64. Also I wonder why time is not set as index by default but It's more a matter of taste

@hoechenberger @dengemann @drammock what do you think?

Start at the raw.to_data_frame() entry point and run the example script from the issue to understand the current time column and index behavior. Add the requested time_format='date' behavior so the exported time values use datetime64, and verify that the existing export behavior remains intact.