Desktop: Fixes #9932: Fix only one CodeMirror 6 content script loaded per plugin

[personalizedrefrigerator]

Summary

Previously, if a plugin registered two CodeMirror content scripts, the IDs used for both content scripts would be the same. This pull request causes the CodeMirror 6 plugin loader to consider both contentScript.pluginId and contentScript.contentScriptId when generating internal IDs.

Fixes #9932.

Testing

1. Load the example plugin from Chore: Plugins: Add CodeMirror 6 plugin tutorial #9933
2. Change the order that the example plugin adds content scripts (add the CodeMirror 5 content script first)
3. Start Joplin with the beta editor enabled
4. Observe that the CodeMirror 6 plugin doesn't load
5. Repeat steps 1-3 with the original order preserved