feat: add optional offset parameter to File.readBytes method

Extend File.readBytes with a second overload accepting an offset argument, allowing reads to start at any byte position from the beginning of the file. The existing single-argument variant now delegates to the new two-argument version with offset 0. Validation for the offset parameter mirrors the existing count checks, aborting on non-numeric, non-integer, or negative values. The underlying C implementation passes the offset directly to uv_fs_read, replacing the previous hardcoded zero. Documentation in the io module markdown is updated with usage examples for both signatures, and the old TODO comment about missing offset support is removed. New test files cover offset-based reads, edge cases (zero offset, past-end offset, zero count with offset), and error conditions for invalid offset arguments.

doc/site/modules/io/file.markdown

@@ -12,7 +12,7 @@ a file descriptor.
 
 ### File.**open**(path, fn)
 
-Opens the file at [path] and passes it to [fn]. After the function returns, the
+Opens the file at `path` and passes it to `fn`. After the function returns, the
 file is automatically closed.
 
     :::wren
@@ -22,7 +22,7 @@ file is automatically closed.
 
 ### File.**read**(path)
 
-Reads the entire contents of the file at [path] and returns it as a string.
+Reads the entire contents of the file at `path` and returns it as a string.
 
     :::wren
     File.read("words.txt")
@@ -33,13 +33,13 @@ whatever encoding the file uses.
 
 ### File.**size**(path)
 
-Returns the size in bytes of the contents of the file at [path].
+Returns the size in bytes of the contents of the file at `path`.
 
 ## Constructors
 
 ### File.**open**(path)
 
-Opens the file at [path] for reading.
+Opens the file at `path` for reading.
 
 ## Methods
 
@@ -61,6 +61,21 @@ Closes the file. After calling this, you can read or write from it.
 
 ### **readBytes**(count)
 
-Reads up to [count] bytes starting from the beginning of the file.
+Reads up to `count` bytes starting from the beginning of the file.
 
-(Allowing an offset to read elsewhere from the file isn't implemented yet.)
+    :::wren
+    // Assume this file contains "I am a file!".
+    File.open("example.txt") {|file|
+      System.print(file.readBytes(6)) //> I am a
+    }
+
+### **readBytes**(count, offset)
+
+Reads up to `count` bytes starting at `offset` bytes from the beginning of
+the file.
+
+    :::wren
+    // Assume this file contains "I am a file!".
+    File.open("example.txt") {|file|
+      System.print(file.readBytes(6, 2)) //> am a f
+    }