It injects into all frames if a tab number is provided (`injectContentScript(1, scripts)`)

I don't remember why I made this choice in https://github.com/fregante/webext-content-scripts/commit/206ab8b11d95d94f9474062b39cef53d35f415c7

https://github.com/fregante/webext-content-scripts/blob/baca20c4d6fb48f3b8a75a2ba0351b2936c6f137/index.ts#L29-L39

This can be avoided by passing a full target instead: `injectContentScript({tabId: 1 frameId:1}, scripts)`

But it's unexpected when calling `injectContentScript(tab, scriptsStraightFromManifest)` like I'm doing in https://github.com/fregante/webext-inject-on-install/pull/3

I have to review how changing this would affect my usage elsewhere.