refactor(client): add type CustomInspectorTab

[skirtles-code]

Currently there's a TS error on this line in SplitScreen.vue:

customPluginId.value = tab.pluginId

Property 'pluginId' does not exist on type 'ModuleBuiltinTab'.

This PR attempts to fix that by introducing a new type, CustomInspectorTab, which extends ModuleBuiltinTab and adds pluginId.

There's a similar error in custom-inspector-tab-view.vue, but it's currently suppressed using @ts-expect-error. That error is also fixed by these changes, so @ts-expect-error has been removed.

I've also introduced some helper functions, isCustomTab and isCustomInspectorTab, in SplitScreen.vue. These should be functionally equivalent to the existing code but they use is to narrow the type of tab, helping to avoid the repeated use of as. In particular, this avoids the need to change customPluginId.value = tab.pluginId to customPluginId.value = (tab as CustomInspectorTab).pluginId.

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