Might still split this up into sub-tasks, but:

• readonly filter text field (from querybar)
• update text field
• how many documents match the filter as an approximate number that will be updated
• preview. For now (this ticket) just put the updated documents there using the same component we use elsewhere. 3 documents should be OK for now. We can change it later. Make it a constant var somewhere. We'll highlight changes or show a diff in another ticket.
• changing the update text updates the preview (same as in aggregations editor)
• if the update is syntactically invalid (ie. does not parse), show the yellow banner just like on aggregations
• if the update is syntactically valid but there is a driver or database error, show the red banner (again just like aggregations)
• start the update with a template, just something like `{ $set: {} }` for now
• If the preview cannot be calculated (yellow or red banner) for now just display text like "No Preview Documents" in place of the preview (again - identical to aggregations)
• If the update is valid and there's a preview, enable the Update button
• Clicking the update button runs the update and closes the modal. (We'll add Updating, Done and Error toasts in a future ticket.)