vim: Add g M motion to go to the middle of a line (#30227)

Adds the "g M" vim motion to go to the middle of the line.

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Co-authored-by: Conrad Irwin <conrad.irwin@gmail.com>

assets/keymaps/vim.json

@@ -152,6 +152,7 @@
       "g end": ["vim::EndOfLine", { "display_lines": true }],
       "g 0": ["vim::StartOfLine", { "display_lines": true }],
       "g home": ["vim::StartOfLine", { "display_lines": true }],
+      "g shift-m": ["vim::MiddleOfLine", { "display_lines": true }],
       "g ^": ["vim::FirstNonWhitespace", { "display_lines": true }],
       "g v": "vim::RestoreVisualSelection",
       "g ]": "editor::GoToDiagnostic",