Add a Text focus areas summary page

[nicoburns]

Summary

Generalizes the focus-areas summary into named sets and adds a second one:

• FocusAreaSet { slug, label, intro, areas } with FOCUS_AREA_SETS (replaces SERVO_FOCUS_AREAS); the route becomes /wpt/focus-areas/{set} with 404 for unknown slugs, and WptFocusAreasPage takes the set's label/intro.
• New data/focus_areas/text.json with the text-layout areas from blitz docs/parley.md (css/CSS2/text, css/CSS2/bidi-text, css/css-text, css/css-inline, css/css-writing-modes, css/css-fonts, css/css-ruby, css/css-text-decor), served at /wpt/focus-areas/text.
• The /wpt root link is now Focus areas: Servo | Text, generated from FOCUS_AREA_SETS.

Verified locally: both pages render scores, unknown slug 404s.

Link to Devin session: https://dioxus.staging.devinenterprise.com/sessions/f2f594019316461cae0fcda15b660ece
Open in Devin Desktop: https://dioxus.staging.devinenterprise.com/desktop/session/f2f594019316461cae0fcda15b660ece?variant=devin-insiders
Requested by: @nicoburns

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