Is your feature request related to a problem? Please describe.
Some shaderpack does not allow to distribute modified versions of them (Some them allow to do so under some circumstances), because they contain original code, so addons often have weird installation methods (Like Euphoria Patches for Complementary)

Describe the solution you'd like
An addon system would allow users to modify shaders without distributing original code (And therefore not breaking licenses). These addons still require to install the base shaderpacks, meaning that all parties would benefit from this method. Horever, the shaderpack's creator can disable this ability, or can put licenses on making addons.

Additional context

When a shader supports addons, A + icon should appear next to the shaderpack. When clicked, it will show a list of installed addons which can be invidually toggled on and off, which are compatible with the shaderpack. To install an addon, you must put the downloaded addon to the .minecraft\shaderaddons folder. If you put the addon in this folder, but doesn't appear, the possible problems include:

• You dont have the shaderpack installed
• Your shaderpack is outdated
• The addon is in an invalid format
• The shaderpack doesn't allow to install addons, or the addon modifies locked parts of shaders