Integrate mentions in the composer (#1799)

* Integrate mentions in the composer:

    - Add `MentionSpanProvider`.
    - Add custom colors needed for mentions.
    - Use the span provider to render mentions in the composer.
    - Allow selecting users from the mentions suggestions to insert a mention.

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Co-authored-by: ElementBot <benoitm+elementbot@element.io>

libraries/designsystem/src/main/kotlin/io/element/android/libraries/designsystem/theme/ColorAliases.kt

@@ -108,6 +108,36 @@ val SemanticColors.pinDigitBg
         Color(0xFF26282D)
     }
 
+val SemanticColors.currentUserMentionPillText
+    get() =  if (isLight) {
+        // We want LightDesignTokens.colorGreen1100
+        Color(0xff005c45)
+    } else {
+        // We want DarkDesignTokens.colorGreen1100
+        Color(0xff1fc090)
+    }
+
+val SemanticColors.currentUserMentionPillBackground
+    get() =  if (isLight) {
+        // We want LightDesignTokens.colorGreenAlpha400
+        Color(0x3b07b661)
+    } else {
+        // We want DarkDesignTokens.colorGreenAlpha500
+        Color(0xff003d29)
+    }
+
+val SemanticColors.mentionPillText
+    get() = textPrimary
+
+val SemanticColors.mentionPillBackground
+    get() =  if (isLight) {
+        // We want LightDesignTokens.colorGray400
+        Color(0x1f052e61)
+    } else {
+        // We want DarkDesignTokens.colorGray500
+        Color(0x26f4f7fa)
+    }
+
 @PreviewsDayNight
 @Composable
 internal fun ColorAliasesPreview() = ElementPreview {