fix: convert signed integer types to unsigned in core and value modules to eliminate -Wsign-compare warnings

- Change `int` to `uint32_t` for iterator indices in `map_iterate` and `string_iterate` to match capacity/length types
- Update `String.length` field type from `int` to `uint32_t` in `wren_utils.h`
- Modify `wrenStringCodePointAt` parameter type from `int` to `uint32_t` and remove redundant negative index assertion
- Convert loop variable in `markMap` from `int` to `uint32_t` to match `map->capacity` type
- Add `-Wsign-compare`, `-Wtype-limits`, and `-Wuninitialized` compiler flags to catch future mismatches

script/wren.mk

@@ -23,7 +23,7 @@ HEADERS   := include/wren.h $(wildcard src/*.h)
 SOURCES   := $(wildcard src/*.c)
 BUILD_DIR := build
 
-CFLAGS := -Wall -Werror
+CFLAGS := -Wall -Werror -Wsign-compare -Wtype-limits -Wuninitialized
 # TODO: Add -Wextra.
 
 # Mode configuration.