feat: make titleText slottable in MultiSelect, ComboBox

[sachinbhutani]

Fix #1747

[metonym]

Looks good – thanks for also applying this to the ComboBox.

One small change I made was to update the conditional wrapping the label, since the label is only rendered if titleText is truthy:

- {#if titleText}
+ {#if titleText || $$slots.titleText}