Add Juneteenth

[kevina]

From https://issues.chromium.org/issues/40700341

[kevina]

ChatGPT analyzes; see #429. Try it for yourself at using this GPT. Note that size 60 is the normal spellchecker dictionary, 70 is the larger, and 80 is the "if it's a valid word" size.

Summary

60

• Juneteenth — 60, not borderline, Signature (US holiday / cultural term). Widely used in mainstream writing; expected not to be flagged.

Signature

60: Juneteenth <n>: Juneteenth's