fix the ability to add a plugin by name instead of path (#13877)

# Description

This plugin fixes the ability to do `plugin add nu_plugin_polars` and
have nushell look in NU_PLUGINS_DIR to find the plugin and add it.

closes #13040

# User-Facing Changes
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crates/nu-cmd-plugin/src/commands/plugin/add.rs

@@ -1,10 +1,9 @@
+use crate::util::{get_plugin_dirs, modify_plugin_file};
 use nu_engine::command_prelude::*;
 use nu_plugin_engine::{GetPlugin, PersistentPlugin};
 use nu_protocol::{PluginGcConfig, PluginIdentity, PluginRegistryItem, RegisteredPlugin};
 use std::sync::Arc;
 
-use crate::util::{get_plugin_dirs, modify_plugin_file};
-
 #[derive(Clone)]
 pub struct PluginAdd;
 
@@ -31,7 +30,7 @@ impl Command for PluginAdd {
             )
             .required(
                 "filename",
-                SyntaxShape::Filepath,
+                SyntaxShape::String,
                 "Path to the executable for the plugin",
             )
             .category(Category::Plugin)
@@ -81,7 +80,6 @@ apparent the next time `nu` is next launched with that plugin registry file.
     ) -> Result<PipelineData, ShellError> {
         let filename: Spanned<String> = call.req(engine_state, stack, 0)?;
         let shell: Option<Spanned<String>> = call.get_flag(engine_state, stack, "shell")?;
-
         let cwd = engine_state.cwd(Some(stack))?;
 
         // Check the current directory, or fall back to NU_PLUGIN_DIRS