feat: add namespace support to Writer

Closes #40

In addition to the new namespace-aware functions, this commit brings several other enhancements:

- The element name has been removed as an argument of `elementEnd`. The writer now keeps track of the current open element automatically.
- Doctests have been added for all public writer functions.
- Doc comments have been added for all public writer functions.

src/Reader.zig

@@ -2236,6 +2236,6 @@ fn addString(reader: *Reader, s: []const u8) !StringIndex {
     return @enumFromInt(start);
 }
 
-fn string(reader: Reader, index: StringIndex) []const u8 {
+fn string(reader: *const Reader, index: StringIndex) []const u8 {
     return std.mem.sliceTo(reader.strings.items[@intFromEnum(index)..], 0);
 }