[Schema] Disallow dots in feature identifiers

[Elchi3]

We should disallow dots in feature identifiers as we are using them to access features and they are indicating a nested structure. (kumascript accesses these nested structures with say {{compat("webextensions.api.devtools.inspectedWindow")}} and thus expects a nested object structure)

"devtools.inspectedWindow": { ... } shouldn't be in the JSON, it should be nested like "devtools": { "inspectedWindow": { ... } } etc.

Helpful resource to create this schema rule: https://spacetelescope.github.io/understanding-json-schema/reference/regular_expressions.html.