feat: add support for custom feeds

src/parser/feedParser.ts

src/ui/PodcastView/PodcastView.svelte

src/ui/settings/PodNotesSettingsTab.ts

src/ui/settings/PodcastQueryGrid.svelte

src/ui/settings/PodcastResultCard.svelte

src/utility/checkStringIsUrl.ts

src/utility/findPlayedEpisodes.ts