Ship the popular themes beside Dark Modern

[vsdudakov]

Six themes now ship: Dark Modern, still the default, and Monokai, Dracula, One Dark Pro, Tokyo Night and Nord — all MIT, each carried as the flattened VS Code theme JSON its extension publishes, under crates/yara-core/assets/themes/. packaging/themes.py (make themes) fetches them afresh, from a raw URL or out of the Open VSX .vsix for themes that only build their JSON at packaging time.

The loader reads that JSON the way VS Code does, which user-dropped themes gain too:

• // comments are allowed — VS Code's own theme files carry them and used to be a hard error.
• A translucent colour is blended onto the editor's ground rather than having its alpha dropped; #db4b4b22 no longer paints a full-strength red diff line.
• Each chrome role has a nearest key to fall back on (diffEditor.removedTextBackground, foreground, editorGroup.border, …). The accent prefers textLink.foreground over focusBorder, which several themes keep as quiet as the panel border, so the live pane's border still reads.
• A selected row whose ground is as bright as the text (Nord's cyan) falls back to the inactive selection, since the row keeps the ordinary foreground.

Tests cover the shipped list, each bundled theme's own palette, tokens and legibility, blending and fallbacks; the themes guide and the settings comment list the six.