Expand shell commands

[superatomic]

Part of #38.
We need to specially handle the $(shell command) syntax.

Details:

• It must translate into the correct form for the given shell. This is easy for Bash, Dash, and Zsh, a bit more complex for Fish, and more challenging for other shells. While we only currently have support for bash, zsh, and fish, we need to make a system that can work for any shell.
• It must allow for escaping the ) character to be able to use them inside of the expression.
• You must able be able to escape the $ to not expand it. This is also relevant to Expand shell variables #53.

[superatomic]

It looks like we need to escape character's ourselves. Instead of worrying about "undoing" the escaping of characters written in a TOML string, we can just use raw TOML strings, which are strings with single quotes (') instead of double quotes (").

We can then write a program to recognize \) as escaping a closing bracket, because \ doesn't escape anything in the TOML representation.

This is a breaking change. We can't really do that much about this. We could try to detect the type of quotations that are used, but that would involve parsing the file and might trigger some false positives (this might be worth it though, if we state that these false positives can happen). Otherwise, the documentation should be changed to use single quotes, and it should be directly stated that using double quotes requires double escaping.

[superatomic]

However, this finally allows us to implement this feature! When this is completed, we can also easily finish the rest of #38.

[superatomic]

To be able to issue warnings, we can read through each line of the file and scan for any line that matches /^[A-Za-z0-9\._]+\s*=\s*\[?\s*"/m. If we find any, we will display a warning that recommends switching to single quotes.

[superatomic]

Also note that the use of double quotes with never be removed, just discouraged for most use cases. This is because after this issue is implemented and merged the new behavior will normally result in having to double escape backslashes while using double quotes.

The warning will either be downgraded it severity or removed in a future version a significant amount of time after the changes in this issue are made. This will probably happen in v1.0.0.

[superatomic]

Update: Warnings will not be issued as that could affect actual use cases.