Add support for slash commands (under Feature Flag) (#6482)

* Add support for slash commands

* Update screenshots

* Rename module `slash` to `slashcommands`

* Rename `SlashCommand` to `SlashCommandService`

* Introduce MsgType in order to send text message with a different msgtype value.

* Format file and add parameter names, add default values and cleanup

* Add isSupported parameter to filter out unsupported yet commands.

* Slash commands: disable suggestions if the feature is disabled.

* Fix sending shrug command.

* Add missing test on SuggestionsProcessor

* Add tests on MessageComposerPresenter about slash command.

* Fix import ordering

* Add missing tests on CommandExecutor

* Add missing tests in MarkdownTextEditorStateTest

* Slash commands: Improve code when sending message with prefix.

* Slash commands: Add support for /unflip

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Co-authored-by: ElementBot <android@element.io>

libraries/textcomposer/impl/build.gradle.kts

@@ -33,6 +33,7 @@ dependencies {
     implementation(projects.libraries.designsystem)
     implementation(projects.libraries.testtags)
     implementation(projects.libraries.uiUtils)
+    implementation(projects.libraries.slashcommands.api)
 
     releaseApi(libs.matrix.richtexteditor)
     releaseApi(libs.matrix.richtexteditor.compose)