Add source map can't concat in a single line

[ahabhgk]

Trying to concat "hello " and "world" in a single line, the minimal reproduce is here:

#[test]
fn reproduce() {
  let mut hello_map = SourceMap::new("/");
  let hello = hello_map.add_source("hello.txt");
  hello_map.set_source_content(hello as usize, "hello ");
  hello_map.add_mapping(
    0,
    0,
    Some(parcel_sourcemap::OriginalLocation {
      original_line: 0,
      original_column: 0,
      source: hello,
      name: None,
    }),
  );
  let mut world_map = SourceMap::new("/");
  let world = world_map.add_source("world.txt");
  world_map.set_source_content(world as usize, "world");
  world_map.add_mapping(
    0,
    0,
    Some(parcel_sourcemap::OriginalLocation {
      original_line: 0,
      original_column: 0,
      source: world,
      name: None,
    }),
  );
  let mut hello_world_map = SourceMap::new("/");
  hello_world_map.add_sourcemap(&mut hello_map, 0);
  hello_world_map.add_sourcemap(&mut world_map, 0);
  println!("{}", hello_world_map.to_json(None).unwrap()); // {"version":3,"sourceRoot":null,"mappings":"ACAA","sources":["hello.txt","world.txt"],"sourcesContent":["hello ","world"],"names":[]}
}

generated map: {"version":3,"sourceRoot":null,"mappings":"ACAA","sources":["hello.txt","world.txt"],"sourcesContent":["hello ","world"],"names":[]} }.
generated string: hello world.

"hello " will loose the mapping, "world" will map to "hello world".

playground link: https://evanw.github.io/source-map-visualization/#MTEAaGVsbG8gd29ybGQxMzIAeyJ2ZXJzaW9uIjozLCJzb3VyY2VSb290IjpudWxsLCJtYXBwaW5ncyI6IkFDQUEiLCJzb3VyY2VzIjpbImhlbGxvLnR4dCIsIndvcmxkLnR4dCJdLCJzb3VyY2VzQ29udGVudCI6WyJoZWxsbyAiLCJ3b3JsZCJdLCJuYW1lcyI6W119