Add Constant type/value properties (#280)

[obi1kenobi]

• add Constant property resolution function

allow reading constant value/expr/is_literal and type (this one serialized to a json string since there is no impl to convert it to FieldValue)

• add Constant property rustdocs example

• add Constant property rustdoc schema

• dep: add serde_json for Constant property Type

• fix clippy

• uglify with rustfmt

• Update src/rustdoc_schema.graphql

Co-authored-by: Predrag Gruevski 2348618+obi1kenobi@users.noreply.github.com

• squeeze doc comment

• add a doc comment note referencing a more comprehensive example

to avoid repeating said example

• add type_ field to AssociatedConstant property resolution function

(temporary in the json string format)

• add type_ field to AssociatedConstant property rustdoc schema

• split schema docs so that each Constant property is documented

• update schema doc comment

• update schema docs trait name

• fix a typo in schema docs

• remove type Assoc/Const property

until trustfall adds proper support for custom scalar types

• remove type Assoc/Const property from schema

• fix a typo in schema docs

Co-authored-by: Predrag Gruevski 2348618+obi1kenobi@users.noreply.github.com

• dep: remove serde_json since Constant property Type was removed

Co-authored-by: Predrag Gruevski 2348618+obi1kenobi@users.noreply.github.com

• remove unstable rustdocs example for Constant properties

Co-authored-by: Predrag Gruevski 2348618+obi1kenobi@users.noreply.github.com

• Update formatting of the schema docs

Co-authored-by: Predrag Gruevski 2348618+obi1kenobi@users.noreply.github.com

• Update AssociatedConstant example to refer to a const generic

• Update Constant test with extra properties

expr/value/is_literal

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Co-authored-by: Predrag Gruevski 2348618+obi1kenobi@users.noreply.github.com