Add step folding to Build Trace screen

What does this MR do?

This adds collapseable sections to gitlab's frontend, as requested in #14664 (closed).

This is implemented by a new trace transformer, and a set of new frontend components; Current documentation in code pointed on the need for frontend rendering of the traces, so that's how I implemented it.

• It allows any build sections with lines in them to be collapsed/expanded
• A collapsed section will show the first line that was logged in it.
• Sections are now allowed to be inside another section, and are not shown if that wrapping section is collapsed (it's a tree)
• Sections have a requirement to have the parent's name as a prefix (e.g. "section" > "section.1" > "section.1.1"). This is an artificial requirement, so this might be changed depending on reviewer input.

The companion merge request in gitlab-runner is available here: gitlab-runner!1194 (closed)

Screenshots & Screencaptures

Top-level sections and unclosed sections are automatically opened, so failed jobs are automatically opened.

What are the relevant issue numbers?

Closes #14664 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer