[Tools] Add an option to compile/archive shaders only if source files changed

[SirLynix]

For easier VS integration, it would be interesting to add a few flags to prevent nzslc and nzsla to compile/archive shaders if nothing changed.

Suggestions:

• --skip-up-to-date: tells the tool to compare the modification time of all its source files vs the output file and to skip compilation/archiving if no source is more recent than the output.

• --skip-unchanged: tells the tool to process as usual but to compare the output with the current file output before writing (to skip writing to a file if its content would be the same). (Implemented in [Tools] Implement --skip-unchanged #75)