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ARROW-14773: [JS] Fix sourcemap paths

This fixes an issue where the source maps point to files in the root directory, not `src`, which is where the sources actually are.

Closes #11741 from domoritz/dom/sourcemaps

Authored-by: Dominik Moritz <domoritz@gmail.com>
Signed-off-by: Dominik Moritz <domoritz@gmail.com>