Add scrollz plugin and ember colorschemes

[vish9812]

Adds two plugins, both pure Lua with no external dependencies, MIT licensed, tested against micro 2.0.15.

scrollz — viewport control. scrollz top / bottom scroll the cursor's line to the edges of the view; micro has Center, but CursorToViewTop/CursorToViewBottom are the inverse, moving the cursor rather than the view. scrollz halfdown / halfup complement HalfPageUp/HalfPageDown, which scroll the view but leave the cursor behind, so Relocate() undoes the scroll on the next cursor movement. Both count display rows rather than buffer lines, so they stay correct under softwrap, and scrollmargin is honoured.

ember — four truecolor colorschemes sharing one accent palette across four canvases: two transparent darks (warm and cool neutrals), one opaque dark, one light. Registered as RTColorscheme runtime files so they show up in set colorscheme completion beside the built-ins, following the same pattern as the existing gotham-colors entry.

Notes for the maintainer

• Added plugins/scrollz.json and plugins/ember.json, and the two channel.json entries pointing at them.
• The Url fields currently point at tagged GitHub archives in my own repos, both verified reachable. Happy to have them re-hosted on the plugins release instead if that is preferred, just say the word and I will update the PR.

Repos: https://github.com/vish9812/micro-scrollz and https://github.com/vish9812/micro-ember