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/core/src/main/kotlin/io/element/android/libraries/core/data/FilterUpTo.kt

@@ -0,0 +1,39 @@
+/*
+ * Copyright (c) 2023 New Vector Ltd
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.element.android.libraries.core.data
+
+/**
+ * Returns a list containing first [count] elements matching the given [predicate].
+ * If the list contains less elements matching the [predicate], then all of them are returned.
+ *
+ * @param T the type of elements contained in the list.
+ * @param count the maximum number of elements to take.
+ * @param predicate the predicate used to match elements.
+ * @return a list containing first [count] elements matching the given [predicate].
+ */
+inline fun <T> Iterable<T>.filterUpTo(count: Int, predicate: (T) -> Boolean): List<T> {
+    val result = mutableListOf<T>()
+    for (element in this) {
+        if (predicate(element)) {
+            result.add(element)
+            if (result.size == count) {
+                break
+            }
+        }
+    }
+    return result
+}