This commit adds attribute completion to the Language Service. It completes
from 3 sources:
1. inputs/outputs of directives currently present on the element
2. inputs/outputs/attributes of directives in scope for the element, that
would become present if the input/output/attribute was added
3. DOM properties and attributes
We distinguish between completion of a property binding (`[foo|]`) and a
completion in an attribute context (`foo|`). For the latter, bindings to
the attribute are offered, as well as a property binding which adds the
square bracket notation.
To determine hypothetical matches (directives which would become present if
a binding is added), directives in scope are scanned and matched against a
hypothetical version of the element which has the attribute.
PR Close#40032
66378ed0ef