Add shell specific options

[superatomic]

Currently, all shells are given the exact same environment variables. While this is optimal for most variables, it is sometimes useful to have different variables be set for certain shells, or to have different shells have different values.

Each shell's specific variables should be represented as a TOML table in a new table shell, with a name corresponding to the shell (eg. [shell.fish]).

Specific shell options should always come after the options that apply to all shells.

Example

EDITOR = "$(which nano)"
CARGO_HOME = "$XDG_DATA_HOME/cargo"

[shell.bash]
HISTFILE = "$XDG_STATE_HOME/bash_history"

[shell.zsh]
HISTFILE = "$XDG_STATE_HOME/zsh_history"
ZSH_CACHE_DIR = "$XDG_CACHE_HOME/oh-my-zsh"