task: Allow obtaining custom task variables from tree-sitter queries (#11624)

From now on, only top-level captures are treated as runnable tags and
the rest is appended to task context as custom environmental variables
(unless the name is prefixed with _, in which case the capture is
ignored). This is most likely gonna help with Pest-like test runners.



Release Notes:

- N/A

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Co-authored-by: Remco <djsmits12@gmail.com>