feat(modals): support stacking modals, remove bootstrap modals dependency (#3456)

* Feature: Stackable modals

* Processed feedback

* fix: use position in modal stack rather than key for modal number

* fix: use correct base z-index

* chore: simplify `className` to `class`

* chore: add `key` attribute to ModalManager element

* fix: backdrop flashing as modals are stacked/unstacked

* chore: simplify modal close process

* docs: add TS overload to indicate deprecated modal opening syntax

Require explicit values for `attrs` and `stackModal` from Flarum 2.0, beginning deprecation from now.

* feat: use stackable modal for forgot password modal above sign in

* chore: explicitly check if modal is open before trying to focus trap

* fix(a11y): add missing  `aria-hidden` on main content when modal open

* fix(a11y): add missing `aria-modal` on modal

* chore: remove test code

* chore: remove dead CSS

* chore: remove overload

* fix: lock page scrolling when modal is open

* fix: strange scrolling behaviour

* chore: convert to JSX

* fix: event listener memory leak

* chore: remove unneeded optional chaining

* fix: incorrect return types

* chore: rewrite backdrop system - use one backdrop for all modals

* docs: typos in comment block

* fix: show backdrop directly below top-most modal

Co-authored-by: Sami Mazouz <sychocouldy@gmail.com>

* chore: format

* fix: use an invisible backdrop for each modal to exit

Signed-off-by: Sami Mazouz <ilyasmazouz@gmail.com>

* chore: remove debugging code

Signed-off-by: Sami Mazouz <ilyasmazouz@gmail.com>

* chore: remove forgotten debug code

Co-authored-by: David Wheatley <david@davwheat.dev>
Co-authored-by: Sami Mazouz <sychocouldy@gmail.com>

framework/core/js/package.json

@@ -6,6 +6,7 @@
     "dependencies": {
         "@askvortsov/rich-icu-message-formatter": "^0.2.4",
         "@ultraq/icu-message-formatter": "^0.12.0",
+        "body-scroll-lock": "^4.0.0-beta.0",
         "bootstrap": "^3.4.1",
         "clsx": "^1.1.1",
         "color-thief-browser": "^2.0.2",
@@ -21,6 +22,7 @@
     },
     "devDependencies": {
         "@flarum/prettier-config": "^1.0.0",
+        "@types/body-scroll-lock": "^3.1.0",
         "@types/jquery": "^3.5.10",
         "@types/mithril": "^2.0.8",
         "@types/punycode": "^2.1.0",