feat(explain): plan trees and the most expensive node for PostgreSQL, MySQL and SQLite (#1164)

[ZhuchkaTriplesix]

Refs #1164

What was wrong

Explain showed the plan as text lines; finding the expensive node in a nested plan meant reading indentation.

Changes

• query_plan.dart: a PlanNode tree and parsers for PostgreSQL EXPLAIN (FORMAT JSON), MySQL EXPLAIN FORMAT=JSON (nested loop of tables) and SQLite EXPLAIN QUERY PLAN rows. Each node has its own cost (cumulative cost minus its children's); the node with the largest own cost is highlighted (QueryPlanParser.hottest).
• Each driver implements explainTree; the base delegate returns none, so a driver without one keeps the text only.
• The result area of an explained query shows a Plan | Text switch. Plan draws the tree top-down with the hottest node outlined and each node's share of the cost as a bar. Text is the previous text plan.
• Tests on recorded plans: a join of three tables gives the right parents and children; rows and costs come through; the own cost subtracts the children; the hottest node is the scan with the largest own cost; SQLite gives no hottest node; MySQL tables become children of the query block.

Not in this PR

• The "click a node for the details panel" part of the issue: nodes show their operation, relation, rows and cost bar, but the other fields are not shown yet (they are parsed into PlanNode.details).
• EXPLAIN ANALYZE and its buffers (the issue asks for it behind an explicit action): not added.
• The acceptance test that a join of three real tables renders with the right structure needs a database; the parser tests cover the structure from recorded plans.

Not verified locally

Tests were not run locally, per the project rule.