Add Subtitle Edit 5 plugin index and Haxor sample plugin

[niksedk]

Summary

Adds the Subtitle Edit 5 plugin index and a sample plugin. SE5's plugin system (subtitleedit#10912) loads plugins as standalone executables that exchange JSON files with Subtitle Edit — a clean break from the SE4 in-process WinForms DLLs, which can't run on the cross-platform Avalonia build.

• se5-plugins.json — the download index read by SE5's Get plugins online... feature. PluginConstants.OnlineIndexUrl points at this file on main.
• se5/Haxor/ — a sample SE5 plugin (under a new se5/ folder to keep it separate from the SE4 plugins). It translates subtitle text to "haxor" (lower-case + look-alike swaps: a→4 e→3 o→0 s→$ i→! c→© h→H k→K n→ñ y→¥), honoring the grid selection. It doubles as a reference implementation of the SE5 plugin contract:
  • plugin.json — manifest
  • PluginContract.cs — request/response DTOs
  • Program.cs — read request → transform → write response → exit 0
  • HaxorTranslator.cs — the transform
  • README.md — build/install/packaging instructions

Notes

• The index downloadUrl points at a se5-v1.0 release that does not exist yet — a Haxor.zip (the published plugin folder, zipped) must be attached to a GitHub release before the in-app installer can fetch it.
• Builds clean with dotnet build (net8.0); verified end-to-end against a sample request.

Test plan

• dotnet publish se5/Haxor/Haxor.csproj -c Release, zip the output + plugin.json, attach to a se5-v1.0 release.
• In SE5: Plugins → Manage plugins... → Get plugins online..., install Haxor, confirm it appears in the Translate area and transforms selected lines.