fix: refactor Modal rendering and clickOutside handling

[ndom91]

• Remove svelte-outclick in favor of our own clickOutside util
• Follow-up to this PR: https://github.com/gitbutlerapp/gitbutler/pull/4010/files
• Tested a few additional modals, including the brand delete one that was causing trouble previously, seems to work again now 👍

Notes

Based on what we discussed:

• Doing the onMount(() => document.appendChild(...)) seems to just change where in the DOM these <dialog /> elements are appended. When using the onMount strategy they're appended to the end of the body, otherwise they're just in line in the dom wherever they the <Modal /> component is being used.
  • Imo this doesn't make much of a difference though because we're (a) still only adding the wrapping <dialog /> element to the DOM initially and none of its children, (b) when the user opens the modal, we render/mount the rest of the body of the <dialog /> and it gets rendered into the top-layer (making it's position in the dom more or less irrelevant).
• Unfortunately it doesn't look like we can wrap the whole <dialog /> with an {#if open ..}, because that <dialog /> element at the very least needs to be in the DOM. The components consuming the <Modal /> freak out if it's not there because of the this binding. If the <dialog /> is not created then the showModal() or close(), etc. methods aren't available.

Todo

• Still need to set enable: !!open for the click outside use: action. This will avoid adding a bunch of unnecessary event handlers on window to listen for clicks to check for Modal's that aren't even open. The use:clickOutside action is on the <form /> element, which is a child of the <dialog />, meaning it's not created and added to the dom until the user opens the modal anyway. And the event listener is removed from the document when the modal is closed 🥳