…uiuc#890)

Add [project.optional-dependencies] to pyproject.toml so users can
install domain-specific dependencies via pip extras:

  pip install pyhealth[graph]   # torch-geometric for GraphCare, KG
  pip install pyhealth[nlp]     # editdistance, rouge_score, nltk

The codebase already uses try/except ImportError with HAS_PYG flags
for torch-geometric, and the NLP metrics define their required
versions in each scorer class. This change exposes those dependencies
through standard Python packaging so pip can resolve them.

Version pins match the requirements declared in the code:
- editdistance~=0.8.1 (pyhealth/nlp/metrics.py:356)
- rouge_score~=0.1.2 (pyhealth/nlp/metrics.py:415)
- nltk~=3.9.1 (pyhealth/nlp/metrics.py:397)
- torch-geometric>=2.6.0 (compatible with PyTorch 2.7)

Closes sunlabuiuc#890

…ture

Move [project.optional-dependencies] from between dependencies and
license (line 49) to after keywords (line 62), before [project.urls].

In TOML, a sub-table header like [project.optional-dependencies]
closes the parent [project] table, so placing it before license and
keywords caused those fields to be excluded from [project]. This
broke CI validation.

Verified with tomllib that all project fields (name, license,
keywords, optional-dependencies, urls) parse correctly under
[project].