fix(devtools-kit): only treat own render property as component definition

[lazerg]

Fixes #1100.

As @skirtles-code pointed out in the issue, the state serializer treats any object with a render method as a Vue component definition, so plain class instances stored in Pinia state (whose class happens to define a render method) are shown as "Unknown component". A component definition keeps render as an own property, whereas a class instance inherits it from the prototype, so this checks for an own render property — real component definitions are still detected, and ordinary objects are left alone.

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