feat: Added align_arena_offset

Added it, it should work, no testing by now so damn, but its fine,
used aformula from stack overflow :)

include/arena.h

@@ -18,6 +18,7 @@ typedef enum {
     ARENA_OUT_OF_SPACE,
     ARENA_NULL_ARG,
     ARENA_INVALID_SIZE,
+    ARENA_INVALID_ALIGN,
 } ArenaErr;
 
 typedef struct {

src/arena.c

@@ -125,4 +125,29 @@ ArenaErr arena_realloc(Arena *arena, size_t new_size) {
     return ARENA_OK;
 }
 
+SizeResult align_arena_offset(Arena *arena, size_t alignment) {
+    if (arena == NULL) {
+        SizeResult err = {
+            .is_valid = false,
+            .err = ARENA_NULL_ARG,
+        };
+        return err;
+    }
+
+    if (alignment < 1) {
+        SizeResult err = {
+            .is_valid = false,
+            .err = ARENA_INVALID_ALIGN,
+        };
+        return err;
+    }
+
+    SizeResult val = {
+        .is_valid = true,
+        .val = ((arena->offset + (alignment - 1)) & ~(alignment - 1)),
+        // Formula for alignment, found at stackoverflow, i came up with one
+        // but it involved 3 divisions, this one is way faster :)
+    };
+    return val;
+}