Debug: limit "object as children" error to elements

[developit]

This should fix preactjs/preact-render-to-string#254 without incurring the performance overhead of converting primitives in JSX text positions to VNodes.

If this works, we can remove these lines from render-to-string.

[coveralls]

Coverage remained the same at 99.531% when pulling 3c7cc27 on debug-limit-object-children into d4089df on master.

[github-actions]

Size Change: -17 B (0%)

Total Size: 52.9 kB

Filename	Size	Change
debug/dist/debug.js	3 kB	-5 B (0%)
debug/dist/debug.module.js	3.01 kB	-5 B (0%)
debug/dist/debug.umd.js	3.08 kB	-7 B (0%)

ℹ️ View Unchanged

Filename	Size	Change
compat/dist/compat.js	3.81 kB	0 B
compat/dist/compat.module.js	3.75 kB	0 B
compat/dist/compat.umd.js	3.87 kB	0 B
devtools/dist/devtools.js	231 B	0 B
devtools/dist/devtools.module.js	240 B	0 B
devtools/dist/devtools.umd.js	315 B	0 B
dist/preact.js	4.05 kB	0 B
dist/preact.min.js	4.08 kB	0 B
dist/preact.min.module.js	4.07 kB	0 B
dist/preact.min.umd.js	4.1 kB	0 B
dist/preact.module.js	4.07 kB	0 B
dist/preact.umd.js	4.12 kB	0 B
hooks/dist/hooks.js	1.49 kB	0 B
hooks/dist/hooks.module.js	1.52 kB	0 B
hooks/dist/hooks.umd.js	1.57 kB	0 B
jsx-runtime/dist/jsxRuntime.js	358 B	0 B
jsx-runtime/dist/jsxRuntime.module.js	324 B	0 B
jsx-runtime/dist/jsxRuntime.umd.js	439 B	0 B
test-utils/dist/testUtils.js	442 B	0 B
test-utils/dist/testUtils.module.js	444 B	0 B
test-utils/dist/testUtils.umd.js	526 B	0 B

_{compressed-size-action}

[marvinhagemeister]

Not sure why I went the other way. Agree that this is a better approach 👍

[developit]

Alright, I just manually confirmed that this change addresses preactjs/preact-render-to-string#254. Will open a PR there.