Improved matrix softfont DECDLD and optimized charsets changes.

[PhMajerus]

Some fixes for #2

• Split the softfont string into more readable header and characters chunks.
• Keep track of currently active charset so we can output font changes only when necessary.

Hopefully this fixes the bus error with the long string, and makes it both more readable and slightly more efficient.

[PhMajerus]

Not sure if you'd consider those improvements though, feel free to discard this and use the puts version if you prefer the black box DECDLD approach and want to avoid the loop.

If you want more details on soft fonts, the VT420 programme's regerence guide contains a better explanation than previous models.
I also have some more VT220 soft fonts in https://github.com/PhMajerus/Documents/tree/main/VT220%20SoftFonts