install.sh should use XDG_DATA_HOME to determine target location for user installs
**Requirements**

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- [x] I have searched the [issues](https://github.com/ryanoasis/nerd-fonts/issues) for my request and found nothing related and/or helpful
- [x] I have searched the [FAQ](https://github.com/ryanoasis/nerd-fonts/wiki/FAQ-and-Troubleshooting) for help
- [x] I have searched the [Wiki](https://github.com/ryanoasis/nerd-fonts/wiki) for help

**Is your feature request related to a problem? Please describe.**
I want to pass a custom install location to the install script (so I can use stow to keep my personal packages organized). I realize this is an uncommon case, but with this simple feature request, I can easily kludge it myself.

**Describe the solution you'd like**
I want the install.sh script to respect `XDG_DATA_HOME` variable for user installations. If not, it should fall back to $HOME/.local/share as per the XDG directory spec.

Basically [line 205 of install.sh](https://github.com/ryanoasis/nerd-fonts/blob/7deaff60d02ad26c38f4f8cc714300c08f598b1e/install.sh#L205) should be

```
usr_font_dir="${XDG_DATA_HOME:-"${HOME}/.local/share"}/fonts"
```

or some equivalent.

(And maybe change the help text for `--install-to-user-path` to "Install fonts to user's font path", removing the unnecessary reference to home.)

This makes more sense according to the standard conventions and configurations of fontconfig.

(This will allow me to override the directory by setting `XDG_DATA_HOME` to `$HOME/.local/stow/nerd-fonts-<ver>/share` before invoking `install.sh`.)

**Describe alternatives you've considered**
Another parameter could be added to the install.sh script to take a directory name. But there's no current options that take parameters, and adding that logic in a bash script can quickly get messy. Supporting XDG_DATA_HOME is simpler, adds support for more complicated environments, and in the spirit of the current `--install-to-user-path` option.

**Additional context**
N/A
