Support ij_kotlin_indent_size in editorconfig

[Copilot]

ij_kotlin_indent_size is an IntelliJ-specific Kotlin language override for indent_size, but ktfmt ignored it entirely. Users who configure indentation via ij_kotlin_indent_size saw no effect on ktfmt formatting.

Changes

• EditorConfigResolver: Register ij_kotlin_indent_size as a recognized property and use it as the highest-priority source for blockIndent, falling back to indent_size → tab_width as before:

  ij_kotlin_indent_size  (new, highest priority)
    → indent_size
    → tab_width (when indent_size = tab)
  

• EditorConfigResolverTest: Two new tests — ij_kotlin_indent_size sets blockIndent, and takes priority over indent_size when both are present.
• docs/editorconfig/: Added ij_kotlin_indent_size to all three example .editorconfig files alongside the existing indent_size.

[Goooler]

Addressed to facebook#604.