Add toner density adjustment option

[jaseg]

I just got a brand new HL-1110, and when printing text set in a serif font, the print quality looked like ass. I found a toner density adjustment in the Windows driver, and after some USB capture and comparison with some PPDs from brother's drivers for other brother printers I added an adjustment to this driver.

Here's one of brother's PPDs for another printer with the same setting:
https://github.com/pathakh/brotherCups/blob/552710c340ebb4c9b54fd0753134e34973a26d24/brother/ppd/BRP7000E_GPL.PPD#L447

...and here's the PJL header of a print job that has the density set to +3 (darker) as sent by brother's latest (five year old) Windows driver:

·%-12345X@PJL 
@PJL SET REPRINT=OFF
@PJL SET HOLD=OFF
@PJL SET USERNAME="jaseg" 
@PJL SET JOBNAME="Developing non-interactive MPC with trusted hardware for enhanced security" 
@PJL JOB NAME="Developing non-interactive MPC with trusted hardware for enhanced security" 
@PJL PRINTLOG ITEM = 1,PRINTER
@PJL PRINTLOG ITEM = 2,Wed,21 Feb 2024 18:15:09
@PJL PRINTLOG ITEM = 3,jaseg
@PJL PRINTLOG ITEM = 4,BIGDATA
@PJL SET JOBTIME = "20240221181509" 
@PJL SET STRINGCODESET=HPROMAN8
@PJL SET ECONOMODE=OFF
@PJL SET MEDIATYPE=REGULAR
@PJL SET DENSITY=3
@PJL SET DEVBIASADJUST=3
@PJL SET LESSPAPERCURL=OFF
@PJL SET FIXINTENSITYUP=OFF
@PJL SET TRANSFERLEVELUP=ON
@PJL SET TRANSFERLEVEL=0
@PJL SET RESOLUTION=600
@PJL SET HQMMODE=OFF
@PJL ENTER LANGUAGE=PCL