Add a variable: search operator over assetsSummary.variableMeasured

[bendichter]

Follow-up to #2882 and #2884, stacked on #2884. Adds a variable: operator over assetsSummary.variableMeasured.

I surveyed the archive before picking this field. Of the fields left in assetsSummary, variableMeasured is populated on about 77% of dandisets and carries 22 distinct values in a 100-dandiset sample, led by ProcessingModule (39), ElectrodeGroup (20), OpticalChannel (17), ImagingPlane (17), Units (15), and ElectricalSeries (14), with SpatialSeries, LFP, PupilTracking, and EyeTracking further down. It answers a genuinely different question from the operators we already have: approach: and technique: describe how the experiment was run, while this describes what is actually in the files, so "which dandisets contain sorted units" or "which have eye tracking" becomes answerable. Against production metadata, variable:LFP returns 22 dandisets, all of them genuinely LFP recordings.

The one structural difference is that variableMeasured is an array of bare strings (NWB neurodata types) rather than objects with a name, so its jsonpath selects the elements themselves and _jsonpath_name_match is renamed to _jsonpath_match, since it no longer always matches a name. variableMeasured being bare strings is guaranteed by dandischema (Optional[List[str]]), and a 100-dandiset production sample across schema versions 0.4.4 through 0.6.4 found no rows using the older PropertyValue shape.

On naming, I went with variable: rather than variable_measured: to match the existing convention, where technique: comes from measurementTechnique and standard: from dataStandard. Underscores show up only in the date operators, where the suffix does real work distinguishing before from after. Happy to rename if you would rather be explicit; it is a one-line change in the parser plus the frontend list.

For the record, the fields I looked at and skipped: numberOfSamples is populated on 8% of dandisets and numberOfCells on none of the 100 I sampled, so operators over either would return almost nothing. numberOfBytes and numberOfFiles are always present, but a size filter already exists in the faceted sidebar. numberOfSubjects is populated on 93% and is worth having, but it is a threshold operator rather than a substring match and is already written up as num_subjects: on the advanced-search-counts branch (#2827), so it belongs there.

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