new string fn

all.h

@@ -683,6 +683,7 @@ fn bool stringDeleteCodepointsBetweenByteOffsetsInclusive(String* s, u32 start,
 fn u32 stringCountLines(String s);
 fn u32 stringLineLen(String s, u32 line_number);
 fn u32 stringLineStartByteOffset(String s, u32 line_number);
+fn bool stringInsertBytesAt(String* s, str bytes, u32 at);
 
 ///// OS-wrapped apis
 void      osInit();

string.c

@@ -466,3 +466,18 @@ fn u32 stringLineStartByteOffset(String s, u32 line_number) {
   return s.length;
 }
 
+fn bool stringInsertBytesAt(String* s, str bytes, u32 at) {
+  u32 len = strlen(bytes);
+  if ((s->capacity - s->length) < len) {
+    return false;
+  }
+  for (u32 i = 0; i < len; i++) {
+    // move the original byte over by `len`
+    s->bytes[at + i + len] = s->bytes[at + i];
+    // copy the new byte in
+    s->bytes[at + i] = bytes[i];
+    s->length += 1;
+  }
+  return true;
+}
+