Check for possible performances issues

[LGala]

When a lot of visibleChars are present, something like this may be particularly slow, because of a lot of list creations

const getInitialPlaceholders = (selectedChar: string): Placeholder[] => {
	const tokens = getVisibleChars();

	const jumpableChars: Char[] = tokens
		.map((char, absoluteIndex) => ({ char, absoluteIndex }))
		.filter(char => isCharJumpable(char, tokens, selectedChar));

	const placeholderGenerator = new IncrementalStringGenerator(jumpableChars.length);

	return jumpableChars.map(char => getPlaceholder(char, placeholderGenerator));
};