option to include sheet files

[SambaGodschynski]

As a composer I want to be able to include other sheet files to have the option to separate parts in several sheet files.

example:

-- Part-A,sheet
[
{
  -- part A
}
]

-- Part-B,sheet
[
{
  r1 :(x10)| 
  -- part B
}
]

[SambaGodschynski]

we don't call it sheet file. we call it part. the vscode extension for example expects that a sheetfile is the only main file.