Fonts: optional ImFont::KerningFn pair-adjust hook

Summary

• Add ImFont::KerningFn / KerningUserData: optional extra advance (pixels at size) between consecutive codepoints.
• Applied in RenderText and ImFontCalcTextSizeEx so layout matches draw.
• NULL keeps the current advance-only path (one null check per glyph). No atlas / loader changes.

Why

ImGui places glyphs by AdvanceX only. Hosts that already have pair data (legacy kern, HarfBuzz GPOS, or a custom table) cannot feed it into chrome/ImGui::Text without wrapping every call. This hook is the smallest seam: the atlas stays a glyph cache; the host owns shaping.

Omar noted in #618 that kerning would be nice as an optional per-font path because text-size calc is a bottleneck. This is that option: default cost is unchanged.

Word-wrap (CalcWordWrapPosition) is not adjusted yet; wrap points can be off by the pair extras on a line. Happy to add that if this shape is acceptable.

Test plan

• Default (KerningFn == NULL): demo / examples look identical.
• Set a callback that returns a known extra for A+V; ImGui::Text(AV) and CalcTextSize(AV) both pick it up.
• Newlines reset the previous codepoint (no pair across lines).