Add generic c40headerlength option to datamatrix encoder

[terryburton]

From https://github.com/metafloor/bwip-js/pull/283

[terryburton]

Update will be based on https://github.com/metafloor/bwip-js/pull/283/commits/0e83d2e7a53da71f6e7b6031f163770906cf96c3

Spec will be:

• c40headerlength option added to datamatrix encoder, and documented (unlike current mailmark option).
• mailmark updated to set c40headerlength=45.
• There will be no separate c40headerdatamatrix encoder. API will be to just call datamatrix with c40headerlength=NN.

[terryburton]

+ @BiosencyAdmin

[terryburton]

Close with 8439d87.

Included in 2023-02-16 release.