The GMT offset and zone fields are not used in calculating the week
number, and `zone` in `struct vtm` is a `VALUE` and should not be cast
to `char *` at least.

Followup: a3f589640fd443deea230c27efd6bdfc92f5817f

Restarting scanning has a fixed cost. Since we build a string
from many small parts, that causes the scan to restart many times.

It's much faster to ignore the coderange, if it's needed later it
will be faster to scan it in one go.

```
compare-ruby: ruby 3.4.0dev (2024-09-04T11:54:06Z opt-strftime-growth 149480f0ff) +YJIT [arm64-darwin23]
built-ruby: ruby 3.4.0dev (2024-09-04T11:55:26Z opt-strftime-coder.. 29a5153cba) +YJIT [arm64-darwin23]
warming up...

|                            |compare-ruby|built-ruby|
|:---------------------------|-----------:|---------:|
|time.strftime("%FT%T")      |      1.870M|    2.225M|
|                            |           -|     1.19x|
|time.strftime("%FT%T.%3N")  |      1.731M|    2.068M|
|                            |           -|     1.19x|
|time.strftime("%FT%T.%6N")  |      1.744M|    2.073M|
|                            |           -|     1.19x|
```

Cumulative with a3f589640fd443deea230c27efd6bdfc92f5817f:

```
compare-ruby: ruby 3.4.0dev (2024-09-04T11:55:26Z opt-strftime-coder.. 29a5153cba) +YJIT [arm64-darwin23]
built-ruby: ruby 3.4.0dev (2024-09-04T12:57:30Z opt-strftime-coder.. 2b938d667a) +YJIT [arm64-darwin23]
warming up...

|                            |compare-ruby|built-ruby|
|:---------------------------|-----------:|---------:|
|time.strftime("%FT%T")      |      1.784M|    2.277M|
|                            |           -|     1.28x|
|time.strftime("%FT%T.%3N")  |      1.504M|    2.056M|
|                            |           -|     1.37x|
|time.strftime("%FT%T.%6N")  |      1.489M|    2.094M|
|                            |           -|     1.41x|
```

Use a classic doubling of capacity rather than only adding
twice as much capacity as is already known to be needed.

```
compare-ruby: ruby 3.4.0dev (2024-09-04T09:21:53Z opt-strftime-2 ae98d19cf9) +YJIT [arm64-darwin23]
built-ruby: ruby 3.4.0dev (2024-09-04T11:46:02Z opt-strftime-growth 586263d6fb) +YJIT [arm64-darwin23]
warming up...

|                            |compare-ruby|built-ruby|
|:---------------------------|-----------:|---------:|
|time.strftime("%FT%T")      |      1.754M|    1.889M|
|                            |           -|     1.08x|
|time.strftime("%FT%T.%3N")  |      1.508M|    1.749M|
|                            |           -|     1.16x|
|time.strftime("%FT%T.%6N")  |      1.488M|    1.756M|
|                            |           -|     1.18x|
compare-ruby: ruby 3.4.0dev (2024-09-04T09:21:53Z opt-strftime-2 ae98d19cf9) +YJIT [arm64-darwin23]
built-ruby: ruby 3.4.0dev (2024-09-04T09:21:53Z opt-strftime-2 ae98d19cf9) +YJIT [arm64-darwin23]
warming up...
```

[Feature #18051]

* See [Feature #17752]
* Using this to detect them:
  git grep -P 'if\s+HAVE' | grep -Pv 'HAVE_LONG_LONG|/ChangeLog|HAVE_TYPEOF'

```
../strftime.c: In function 'rb_strftime_with_timespec':
../strftime.c:392:39: warning: comparison is always false due to limited range of data type [-Wtype-limits]
  392 |                         if (vtm->wday < 0 || vtm->wday > 6)
      |                                       ^
../strftime.c:403:39: warning: comparison is always false due to limited range of data type [-Wtype-limits]
  403 |                         if (vtm->wday < 0 || vtm->wday > 6)
      |                                       ^
```

In RFC 3339, -00:00 is used for the time in UTC is known, but the
offset to local time is unknown.  Support that representation by
`-` flag for `z`.