imgui: add version 1.87

[toge]

Specify library name and version: imgui/1.87

---

• I've read the guidelines for contributing.
• I've followed the PEP8 style guides for Python code in the recipes.
• I've used the latest Conan client version.
• I've tried at least one configuration locally with the
  conan-center hook activated.

[conan-center-bot]

All green in build 1 (155fc15bb345084593bf89cf44ba046b23dc9991):

• imgui/1.80@:
  All packages built successfully! (All logs)

• imgui/1.76@:
  All packages built successfully! (All logs)

• imgui/1.75@:
  All packages built successfully! (All logs)

• imgui/1.79@:
  All packages built successfully! (All logs)

• imgui/1.83@:
  All packages built successfully! (All logs)

• imgui/1.74@:
  All packages built successfully! (All logs)

• imgui/1.77@:
  All packages built successfully! (All logs)

• imgui/1.86@:
  All packages built successfully! (All logs)

• imgui/1.78@:
  All packages built successfully! (All logs)

• imgui/1.82@:
  All packages built successfully! (All logs)

• imgui/1.81@:
  All packages built successfully! (All logs)

• imgui/1.85@:
  All packages built successfully! (All logs)

• imgui/1.84.2@:
  All packages built successfully! (All logs)

• imgui/1.84.1@:
  All packages built successfully! (All logs)

• imgui/1.87@:
  All packages built successfully! (All logs)