Add Typewriter effect SE5 plugin (Avalonia)

[niksedk]

Summary

• Adds a new SE5 plugin Typewriter effect under `se5/TypewriterEffect/`, addressing SE5 Typewriter Mode subtitleedit#10942.
• Each selected subtitle line is split into N short lines that progressively reveal the text character by character, optionally followed by a final line that holds the fully-typed text for a configurable end delay before the original end time. Logic is a faithful port of the SE4 `EffectTypewriter` form.
• Built as an Avalonia 11 / .NET 8 desktop app — single settings dialog with a numeric input for the end delay, plus OK / Cancel. Theme variant (Dark / Light) is picked from `request.theme` so the plugin matches Subtitle Edit's appearance.
• End-delay is persisted via the plugin Settings round-trip.
• Inline SRT parser/serializer; ASSA overrides and ` <font ...>` tags are preserved on the exploded paragraphs.

Build

• `.github/workflows/typewriter.yml` mirrors the existing `haxor.yml`: matrix self-contained publish for `win-x64 / win-arm64 / linux-x64 / linux-arm64 / osx-x64 / osx-arm64`, plugin.json rewritten per-platform with the `executables` block, six zips uploaded as artifacts. On manual dispatch with a tag, attaches all six to a GitHub release.
• `se5-plugins.json` extended with a Typewriter-effect entry pointing at the planned `se5-typewriter-v1.0` release assets via the per-platform `downloads` map (matches the schema added in Per-platform plugin downloads in the index subtitleedit#10999).

Notes / caveats

• Self-contained single-file Avalonia + .NET 8 is ~98 MB per platform — bigger than Haxor (~30 MB) but expected.
• Skipped a live animated preview from SE4 (was a winforms-specific Timer + RichTextBox); the dialog only exposes the settings. Could be added later.
• The release zips do not exist yet; `se5-plugins.json` is staged for when `workflow_dispatch` is run with tag `se5-typewriter-v1.0`.

Test plan

• CI matrix build passes on all six RIDs.
• Locally: select a few lines in SE5, run the plugin, verify the dialog appears in the same theme as SE.
• OK: those lines are exploded into character-reveal paragraphs; end-delay > 0 holds the full line.
• Cancel: subtitle unchanged.
• Re-running shows the last end-delay value.

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