Warn on non-pure bricks in renderer pipelines

[twschiller]

Context

• Users don't have control over when renderer pipelines run (e.g., the sidebar pipeline)

Implementation Sketch

• Create an Analysis Visitor that checks the isPure property of a brick against the
• Annotate with a warning if an impure brick appears in a renderer context
• If the impure brick is the form-modal brick, recommend the Custom Form brick instead

Discussion

• How accurate are our isPure annotations/logic? Do we need to carve out some exceptions? Change the warning level?

Related

• https://github.com/pixiebrix/pixiebrix-extension/issues/4025