feat: cache common StringDoc types (#1494)

Add `StringDoc.Create` to reuse common `StringDoc` types where possible.
Saving 1.7 MB, 2% of memory usage.

### Most common `StringDoc` types (total 97,000)
![Screenshot 2025-02-18
201936](https://github.com/user-attachments/assets/edff4d0b-f8ef-42d6-82a4-a3ec500faa87)

- I did most of the common single character symbols, it might be worth
adding other common symbols like `int`, `string`, `true`, `var`, `new`
etc. The switch statements of this size are lowered into an efficient
hash/binary tree so I doubt it makes a meaningful performance impact.
- Is it worth exploring caching all new `StringDoc` types? There are a
total of 674 unique `StringDoc` symbols in the example benchmark, I
wonder how many unique symbols are in a real world 1M loc project.
Perhaps a cache for each file or using a static `ConditionalWeakTable`.
I can see this causing a memory leak if implemented incorrectly😅 

- A future change could look into creating a `SpanDoc` type that
contains `SyntaxToken.Span`, this is used to copy the relevant section
of the original code into the new text without allocating.

Co-authored-by: Bela VanderVoort <[email protected]>

Author: TimothyMakkison

Src/CSharpier/DocTypes/Doc.cs

@@ -7,10 +7,7 @@ public override string ToString()
         return DocSerializer.Serialize(this);
     }
 
-    public static implicit operator Doc(string value)
-    {
-        return new StringDoc(value);
-    }
+    public static implicit operator Doc(string value) => StringDoc.Create(value);
 
     public static NullDoc Null => NullDoc.Instance;