slash_commands_example: Improve setup instructions in README (#26217)

Chris Boette created 

This PR improves the setup instructions for the slash-commands-example
extension by:

1. Replacing the `sed` command with a more reliable approach that
completely replaces the Cargo.toml file.

2. Explicitly showing how to create a standalone extension with a
properly configured Cargo.toml file that:
   - Uses `edition = "2021"` instead of `edition.workspace = true`
   - Doesn't include `publish.workspace = true`
   - Doesn't include the `[lints]` section

This change addresses an issue where the extension wouldn't work when
copied as a standalone project due to workspace references that are only
valid when the extension is built as part of the main Zed repository.

The updated instructions provide a clear, reliable path for developers
to create their own Zed extensions based on the slash commands example.

Release Notes:

- N/A

Change summary

extensions/slash-commands-example/README.md | 21 ++++++++++++++++++---
1 file changed, 18 insertions(+), 3 deletions(-)

Detailed changes

extensions/slash-commands-example/README.md 🔗 

@@ -20,7 +20,22 @@ cp -RL zed/extensions/slash-commands-example .
 
 cd slash-commands-example/
 
-sed -i '' '/\[lints]/,/^$/s/^workspace/#&/' Cargo.toml
+# Update Cargo.toml to make it standalone
+cat > Cargo.toml << EOF
+[package]
+name = "slash_commands_example"
+version = "0.1.0"
+edition = "2021"
+license = "Apache-2.0"
+
+[lib]
+path = "src/slash_commands_example.rs"
+crate-type = ["cdylib"]
+
+[dependencies]
+zed_extension_api = "0.1.0"
+EOF
+
 curl -O https://raw.githubusercontent.com/rust-lang/rust/master/LICENSE-APACHE
 echo "# Zed Slash Commands Example Extension" > README.md
 echo "Cargo.lock" > .gitignore
@@ -40,7 +55,7 @@ zed $_
 
 1. Open the command palette (`cmd-shift-p` or `ctrl-shift-p`).
 2. Launch `zed: install dev extension`
-3. Select the `slash-commands-example` folder created above
+3. Select the extension folder created above
 
 ## Test
 
@@ -50,7 +65,7 @@ Open the assistant and type `/echo` and `/pick-one` at the beginning of a line.
 
 Open the `extensions.toml` file and set the `id`, `name`, `description`, `authors` and `repository` fields.
 
-Rename `slash-commands-example.rs` you'll also have to update `Cargo.toml
+Rename `slash-commands-example.rs` you'll also have to update `Cargo.toml`
 
 ## Rebuild