Enable stylelint rule no-unknown-custom-properties (#28473)

* Enable stylelint rule no-unknown-custom-properties

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

* Fix cpd css vars

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

* Remove dead styling

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

* Remove invalid css

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

* Fix comments

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

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Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

res/css/views/rooms/_BasicMessageComposer.pcss

@@ -7,6 +7,11 @@ Please see LICENSE files in the repository root for full details.
 */
 
 .mx_BasicMessageComposer {
+    /* These are set in Javascript */
+    --avatar-letter: "";
+    --avatar-background: unset;
+    --placeholder: "";
+
     position: relative;
 
     .mx_BasicMessageComposer_inputEmpty > :first-child::before {

res/css/views/rooms/_EventTile.pcss

@@ -1017,16 +1017,6 @@ $left-gutter: 64px;
     visibility: visible;
 }
 
-/* Inverse of the above to *disable* the animation on any indicators. This approach */
-/* is less pretty, but is easier to target because otherwise we need to define the */
-/* animation for when it's shown which means duplicating the style definition in */
-/* multiple places. */
-.mx_EventTile:not(:hover):not(.mx_EventTile_actionBarFocused):not([data-whatinput="keyboard"] :focus-within) {
-    &:not(:focus-visible:focus-within) .mx_MessageActionBar .mx_Indicator {
-        animation: none;
-    }
-}
-
 .mx_EventTile[data-shape="ThreadsList"],
 .mx_EventTile[data-shape="Notification"] {
     --topOffset: $spacing-12;

res/css/views/rooms/wysiwyg_composer/components/_Editor.pcss

@@ -7,6 +7,11 @@ Please see LICENSE files in the repository root for full details.
 */
 
 .mx_WysiwygComposer_Editor_container {
+    /* These are set in Javascript */
+    --avatar-letter: "";
+    --avatar-background: unset;
+    --placeholder: "";
+
     @keyframes visualbell {
         from {
             background-color: $visual-bell-bg-color;