Add Theme Abilities (tracking issue)

[mmaattiiaass]

Add Theme Abilities:

core/get-active-theme
Returns information about the currently active theme, optionally filtered to a subset of fields.

core/get-theme-info
Returns information about a specific installed theme looked up by its stylesheet slug, optionally filtered to a subset of fields.

core/get-themes
Returns a list of all installed themes (optionally filtered by active/inactive status) along with the slug of the currently active one.

core/activate-theme
Activates an installed theme by its stylesheet slug and reports the previous and new active stylesheet.

[gziolo]

Thanks for working on these additions. They look directionally useful and align with the earlier discussion in ​https://github.com/WordPress/ai/issues/40 where this area started to take shape.

Before adding several new abilities, I wonder if we should discuss the right level of granularity. If we introduce many small, similar abilities, selection may become harder for clients and agents. Could some of these be consolidated into fewer abilities with parameters, and can we define criteria for when a separate ability is warranted?