Migrate command execution to structured argv lists across the plugin, replacing shell-string command construction with `vim.system` argv execution. The runner API now exposes argv-based execution as the default path:
- `runner.execute_command` -> `runner.execute`
- `runner.execute_command_async` -> `runner.execute_async`
- `runner.execute_command_raw` -> `runner.execute_raw`
- `runner.execute_command_raw_async` -> `runner.execute_raw_async`
- `runner.execute_command_sync` was removed; use `runner.execute` and handle the result directly
BREAKING CHANGES:
- `cmd.new({ args = ... })` now requires `args` as `string[]` argv tokens, not a single string.
- `cmd.log({ ... })` now uses `raw_flags` as `string[]` for raw passthrough flags; `raw = "..."` is no longer supported.
- Direct users of `jj.core.runner` must migrate from the old `execute_command*` string helpers to the new argv-based `execute*` helpers.
Add split command that allows splitting a change into multiple revisions
from the log buffer or via :J split. Supports --parallel, --message,
--fileset, and --ignore-immutable flags. Includes immutability checks
with user confirmation and empty change detection.
