fix: nirs convert only auto accepted

[sappelhoff]

PR Description

fixes https://mne.discourse.group/t/write-raw-bids-and-snirf/5843

without this fix, passing a NIRS file with a format other than "auto" to write_raw_brainvision will run into an issue at this line:

mne-bids/mne_bids/write.py

Line 1905 in a552a53

elif format not in CONVERT_FORMATS[datatype]:

with this fix, a sensible error is raised, namely: users should use format="auto" with NIRS files.

Merge checklist

Maintainer, please confirm the following before merging.
If applicable:

• All comments are resolved
• This is not your own PR
• All CIs are happy
• PR title starts with [MRG]
• whats_new.rst is updated
• New contributors have been added to CITATION.cff
• PR description includes phrase "closes <#issue-number>"

[codecov]

Codecov Report

Merging #1092 (0f170c5) into main (a552a53) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1092   +/-   ##
=======================================
  Coverage   95.20%   95.20%           
=======================================
  Files          24       24           
  Lines        3860     3861    +1     
=======================================
+ Hits         3675     3676    +1     
  Misses        185      185           

Impacted Files	Coverage Δ
mne_bids/config.py	97.67% <100.00%> (+0.02%)	⬆️

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[agramfort]

@sappelhoff can I ask you to update what's new and maybe add a test?

🙏

[sappelhoff]

@agramfort, done!