fix regex for shader define: must have at least one whitespace (#7754)

# Objective

- Nothing render
```
ERROR bevy_render::render_resource::pipeline_cache: failed to process shader: Invalid shader def definition for '_import_path': bevy_pbr
```

## Solution

- Fix define regex so that it must have one whitespace after `define`

Author: mockersf

crates/bevy_render/src/render_resource/shader.rs

@@ -407,7 +407,7 @@ impl Default for ShaderProcessor {
             else_ifdef_regex: Regex::new(r"^\s*#\s*else\s+ifdef\s*([\w|\d|_]+)").unwrap(),
             else_regex: Regex::new(r"^\s*#\s*else").unwrap(),
             endif_regex: Regex::new(r"^\s*#\s*endif").unwrap(),
-            define_regex: Regex::new(r"^\s*#\s*define\s*([\w|\d|_]+)\s*([-\w|\d]+)?").unwrap(),
+            define_regex: Regex::new(r"^\s*#\s*define\s+([\w|\d|_]+)\s*([-\w|\d]+)?").unwrap(),
             def_regex: Regex::new(r"#\s*([\w|\d|_]+)").unwrap(),
             def_regex_delimited: Regex::new(r"#\s*\{([\w|\d|_]+)\}").unwrap(),
         }