feat: allow including external code snippets (#328)

This allows adding snippets that pull in an external source file.
Attributes still work like you'd expect. Examples:

A rust file that allows execution:
~~~
```file +exec +line_numbers
path: snippet.rs
language: rust
```
~~~

A mermaid script that is rendered like you'd expect
~~~
```file +render
path: snippet.mmd
language: mermaid
```
~~~

Relates to #307

src/custom.rs

@@ -1,7 +1,7 @@
 use crate::{
     input::user::KeyBinding,
-    markdown::elements::SnippetLanguage,
     media::{emulator::TerminalEmulator, kitty::KittyMode},
+    processing::code::SnippetLanguage,
     GraphicsMode,
 };
 use clap::ValueEnum;

src/execute.rs

@@ -2,7 +2,7 @@
 
 use crate::{
     custom::LanguageSnippetExecutionConfig,
-    markdown::elements::{Snippet, SnippetLanguage},
+    processing::code::{Snippet, SnippetLanguage},
 };
 use once_cell::sync::Lazy;
 use os_pipe::PipeReader;
@@ -244,7 +244,7 @@ impl ProcessStatus {
 #[cfg(test)]
 mod test {
     use super::*;
-    use crate::markdown::elements::SnippetAttributes;
+    use crate::processing::code::SnippetAttributes;
 
     #[test]
     fn shell_code_execution() {