new rule: no-duplicate-spread-property

[ajafff]

Checklist

• Fixes: add rule to warn about properties that are overridden by object spread #184
• Added or updated tests / baselines
• Documentation update

Overview of change

Adds a rule to detect properties in object literals that are overridden later by a spreaded object.
It also detects spreaded objects that only contain properties that are overridden later.

TBD

• is there a better name for the rule?
• how to handle properties that are not optional, but may contain undefined? The compiler doesn't allow assigning optional properties to non-optional ones, but maybe some libraries typings are not correct.

[ajafff]

Two things to note here:

• a better name would be no-duplicate-spread-property
• this rule will never work 100% correct. object spread only copies own properties. properties and methods on the prototype not copied although they part of the type (they cannot be distinguished from own properties)

[ajafff]

Some more details on the design limitation regarding own properties vs. prototype properties: microsoft/TypeScript#13148 (comment)

TODO: only omit class methods, but include properties with call signatures. See microsoft/TypeScript#13365 for more details