[ENH] Allow mod-* entity to relate physio data to MRI data in anat/ (bids-standard#1991)

* [SCHEMA] Allow `mod-*` to relate physio data to MRI data in `anat/`

`mod` seems to be more appropriate, but was also not yet supported.

* Update task.yaml

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Co-authored-by: Chris Markiewicz <[email protected]>

Author: tstoeter

src/schema/rules/files/raw/task.yaml

@@ -112,6 +112,7 @@ timeseries__anat:
     echo: optional
     part: optional
     chunk: optional
+    modality: optional
 
 timeseries__func:
   $ref: rules.files.raw.task.timeseries