FreeType: Add variable font axes support

[PeterDimov]

This PR adds support for configuring variable font axes (weight, width, slant, optical size, etc.) when using the FreeType font loader.

Features

• ImFontConfigVarAxes - New struct to specify axis values in ImFontConfig
• IM_FONT_TAG() - Macro to create OpenType axis tags (e.g., IM_FONT_TAG('w','g','h','t') for weight)
• ImGuiFreeType::GetFontAxes() - Utility to query axis metadata from font files
• New example: example_win32_directx11_freetype - Interactive font preview demonstrating variable font support

Usage

// Query axes from font file
ImFontVarAxisInfo axes[16];
int axis_count = ImGuiFreeType::GetFontAxes("RobotoFlex.ttf", axes, 16);

// Configure axes when adding font
ImFontConfigVarAxis my_axes[] = {
    { IM_FONT_TAG('w','g','h','t'), 700.0f },  // Bold weight
    { IM_FONT_TAG('w','d','t','h'), 100.0f },  // Normal width
};
ImFontConfig config;
config.VarAxes.Axes = my_axes;
config.VarAxes.AxesCount = IM_ARRAYSIZE(my_axes);
io.Fonts->AddFontFromFileTTF("RobotoFlex.ttf", 16.0f, &config);

###Screenshots
These show the example app with Bahnschrift and RobotoFlex variable fonts and axes adjustments:

###Notes

• Requires IMGUI_ENABLE_FREETYPE and FreeType library
• No changes to non-FreeType code paths