refactor(applet): fix linting problems with props

[skirtles-code]

Linting currently shows 3 warnings:

• https://github.com/vuejs/devtools/actions/runs/23250491530/job/67590858933#step:8:11

/home/runner/work/devtools/devtools/packages/applet/src/components/state/ChildStateViewer.vue
Warning:   7:3  warning  Prop "depth" should be optional  vue/no-required-prop-with-default

/home/runner/work/devtools/devtools/packages/applet/src/components/tree/TreeViewer.vue
Warning:   12:3  warning  Prop "depth" should be optional    vue/no-required-prop-with-default
Warning:   13:3  warning  Prop "withTag" should be optional  vue/no-required-prop-with-default

This PR addresses those 3 warnings.

The current prop configurations lead to type checking errors in other files, which are also fixed by these changes.

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commit: 29e6bea