fix: harden extraction output and error handling

[reobin]

extraction output is now deterministic, and failures are loud instead of silent.

• highlight colors always come back as uppercase #RRGGBB strings, with only the curated attributes kept; cterm mode reads the cterm attributes table
• extract() validates its input and raises errors for an invalid output path, a bad colorscheme list, and failed default highlights, instead of printing and returning
• extract() returns the extracted data table
• the colorschemes list is deduplicated, and VCSExtract with no arguments now falls back to all installed colorschemes
• empty extraction output encodes as {} instead of []
• the readme documents the output json shape

this is the plugin-side counterpart to vimcolorschemes/worker#95, which can then rely on always-hex output and thrown errors rather than normalizing and guarding locally.