yaml/highlights.scm: Treat plain value with string face

[cldotdev]

The plain value (without quotes) in a YAML file is treated as a string. Therefore, its font face should match that of the quoted string

Original appearance:

After the fix, it looks like:

[meain]

I think not highlighting it is how yaml-mode does it by default. Looking at neovim implementation, it seems to be the same as well. And so, I personally think we should leave it as is.

[cldotdev]

The plain text is highlighted by default in VS Code. However, I agree to leave it as it is if the package strictly follows the original design of yaml-mode.

[meain]

I agree to leave it as it is if the package strictly follows the original design of yaml-mode.

The package AFAICT does not strictly follow the design of yaml-mode. That said, I personally don't think deviating from it brings us anything more in this specific case. I don't have any strong opinions on this. I'll leave it to @ubolonton on this one.