Add editor initialization script

[shriram-svg]

Fixes #3119.

Summary:

• Adds an Editor initialization script preference backed by CodeMirror in the UI settings tab.
• Runs the saved script after CodeMirror/Vim setup for full and split markdown editors.
• Exposes CodeMirror, editor, and cm aliases to the script and catches startup errors.

Tests:

• ./node_modules/.bin/jest tests/lib/editor-initialization-script.test.js --runInBand
• ./node_modules/.bin/eslint browser/components/CodeEditor.js browser/main/lib/ConfigManager.js browser/lib/editorInitializationScript.js tests/lib/editor-initialization-script.test.js
• ./node_modules/.bin/prettier --check browser/components/CodeEditor.js browser/components/MarkdownEditor.js browser/components/MarkdownSplitEditor.js browser/main/lib/ConfigManager.js browser/main/modals/PreferencesModal/UiTab.js browser/lib/editorInitializationScript.js tests/lib/editor-initialization-script.test.js
• git diff --check

Note: direct linting of UiTab.js/MarkdownSplitEditor.js is still blocked by the repository's existing react/no-unknown-property errors around the styleName CSS modules pattern.