feat: add --prepend option for pixi project channel add

[mrswastik-robot]

trying to fix #2349

Example:

If the current pixi.toml has:

[project]
channels = ["conda-forge"]

After running pixi project channel add pytorch --prepend , the channels list will become:

[project]
channels = ["pytorch", "conda-forge"]

unit test regarding the above functionality is passing:

this is my first time trying to contribute to pixi, and I am not that good in Rust too, so any kind of help will be very much appreciated. I will be happy to make requested code changes for this PR.

[ruben-arts]

Thank you, could you add it to the cli documentation? (docs/reference/cli.md)

[mrswastik-robot]

Thank you, could you add it to the cli documentation? (docs/reference/cli.md)

hii @ruben-arts , I have done it, can you have a look into it

[ruben-arts]

Great nice work!