Regularize feature namespaces for CSS, HTML, HTTP, and JavaScript

[ddbeck]

Several of BCD's namespaces appear in surprising or incorrect places. This makes it more difficult to consume the data and to determine the type or meaning of the data of the features inside them. We ought to clean them up so that BCD's API is more consistent and easier to reason about between versions.

In general, the name spaces highlighted in the tree below need one of these fixes:

• A subfeature ought to be promoted to parent feature (e.g., CSS properties where a new flex or grid contexts were added to an existing property) or moved elsewhere (maybe the SVG features?)
• The name space ought to be squashed onto its children (e.g., http.headers.csp)
• Feature data ought to be added (e.g., javascript.builtins.Math)

Tree

The bolded namespaces are irregular and need some sort of fix in a new PR.

• css
  • properties
    • align-content Add __compat entries for all flex context CSS properties (+ fix discrepancies) #17130
    • align-items Add __compat entries for all flex context CSS properties (+ fix discrepancies) #17130
    • align-self Add __compat entries for all flex context CSS properties (+ fix discrepancies) #17130
    • break-after Add __compat entries for all flex context CSS properties (+ fix discrepancies) #17130
    • break-before Add __compat entries for all flex context CSS properties (+ fix discrepancies) #17130
    • break-inside Add __compat entries for all flex context CSS properties (+ fix discrepancies) #17130
    • column-gap Add __compat entries for all flex context CSS properties (+ fix discrepancies) #17130
    • gap Add __compat entries for all flex context CSS properties (+ fix discrepancies) #17130
    • justify-content Add __compat entries for all flex context CSS properties (+ fix discrepancies) #17130
    • justify-items Add __compat entries for all flex context CSS properties (+ fix discrepancies) #17130
    • justify-self Add __compat entries for all flex context CSS properties (+ fix discrepancies) #17130
    • row-gap Add __compat entries for all flex context CSS properties (+ fix discrepancies) #17130
• html
  • elements
    • menu
      • type Move html.elements.menu.type.type_menu up a level #16981
• http
  • headers
    • csp Move Content-Security-Policy header up a level #16980
• javascript
  • builtins
    • Math (fixed by Add Math __compat feature; sort #13903)
  • operators
    • spread Add __compat to spread operator #16979

Similar work needs to be done for SVG and WebExtensions data. However, those are more complicated (it's not clear if content changes must also happen for them), so I didn't include them in the tree above.