Support adding custom attribute in extract from page text/attribute selector

[twschiller]

User Story

As a mod developer, I want to choose which attribute to read in the Extract from Page brick, even if there are not attribute examples loaded

Motivation

In some cases, e.g. working with iframes or modifying a mod without access to the page (e.g., modifying a mod while you're on a remote session), it can be impossible to generate the examples required to select the attribute

Screenshots

Acceptance Criteria

• The "Extract" dropdown allows the user to add an attribute name

Related Code

• https://github.com/pixiebrix/pixiebrix-extension/blob/bc985fb397f5a289a883668aa2191e82335253cb/src/bricks/transformers/jquery/JQueryReaderOptions.tsx#L304-L304