Allow custom distro icon

[fourpastmidnight]

Requirements (please check off with 'x')

• I have read the Contributing Guidelines
• I have read or at least glanced at the FAQ
• I have read or at least glanced at the Wiki

What does this Pull Request (PR) do?

Adds a configuration variable, g:WebDevIconsDistro to allow users to specify their distribution's icon. Several popular distributions are not supported "out of the box" by this plugin, i.e., Fedora, Suse Linux, etc. This PR also adds my particular distro: Manjaro Linux.

How should this be manually tested?

I use vim-plug as my vim plugin manager. I have vim-devicons plugin listed last, as directed.
I use let g:WebDevIconsDistro = "\uF30B" in my .vimrc to force vim-devicons to use my selected icon. This produced the following result:

Next, I removed the custom distro icon setting from my .vimrc to test that the addition of Manjaro as a detected Linux distribution is working. This resulted in:

Any background context you can provide?

N/A

What are the relevant tickets (if any)?

Fixes #445

Screenshots (if appropriate or helpful)

N/A

[HoneyBearCodes]

I previously used the Material Icon plugin in VSCode, and I loved how it not only provided file icons but also customized folder icons based on their types—like styles, tests, utils, views, and models. I think this feature really enhances organization and visibility. How can I submit a feature request to implement something similar?