feat: add treesitter syntax highlighting and word-level diff to split view

[KEY60228]

Summary

• Strip the +/-/ prefix from rendered diff lines so the split buffers contain pure code — the add/remove state is already conveyed by the line highlights and the colored inline line numbers
• Enable treesitter-based language syntax highlighting in the diff panes: the language is detected from the file path (vim.filetype.match + vim.treesitter.language.get_lang) and the highlighter is attached with pcall(vim.treesitter.start, ...), falling back silently to no highlighting when no parser is installed (keeps the plugin zero-dependency); the highlighter is stopped before re-rendering a different file to avoid stale parsers
• Add word-level (intra-line) diff highlighting: for each positionally paired remove/add line, the common prefix and suffix are scanned byte-wise (backing off to UTF-8 character boundaries) and the differing middle span is highlighted on both sides via new ReviewThemWordAdd / ReviewThemWordDelete groups (linked to DiffText, theme-friendly)
• Add a word_diff = true config option to disable word-level highlighting; documented in README and :help reviewthem-configuration

Implementation notes

• Word diff spans are computed in build_split_content() and carried through the line map (entry.word_diff = {start_col, end_col}), then applied as hl_group extmarks with end_col in apply_split_decorations() (priority 110, above treesitter's 100)
• Pure adds/removes with no counterpart and identical pairs get no word diff mark
• Diff backgrounds still show through treesitter highlighting since line_hl_group only contributes the background
• No code relied on the old 1-column prefix offset (cursor context and keymaps are row-based), so stripping it required no other adjustments

Test plan

• Headless load check: nvim --headless -c "set rtp+=." -c "lua require('reviewthem').setup()" -c "q"
• Headless functional check (scratch script via nvim --headless -l): fed a fake DiffFile through render_file and asserted prefix-free buffer lines, empty padding lines, expected word diff spans (including a héllo→hállo case verifying spans don't split multibyte chars), no span on unpaired removes, active treesitter highlighter on both buffers, working cursor context, and that word_diff = false removes the marks
• Open a review session on a Lua/TS file with a parser installed — verify code is syntax highlighted and changed spans within modified lines stand out
• Open a file with no parser installed — verify the diff renders normally with no errors

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