Allow ssh and sshconfig tags for ssh config syntax highlighting.

[coolaj86]

Describe the enhancement

Support ```sshconfig, ```ssh, and ```ssh-config tags for sshconfig highlight blocks.

Support in the Wild

At the time of the previous issue discussion ssh config (https://github.com/github-linguist/linguist/issues/3450), the Include directive for ssh config was not well known, nor widely adopted.

However, now it is supported in macOS, Ubuntu, and many other Linuxes (anything with OpenSSH 8.x or higher): https://man.openbsd.org/ssh_config.5

Although the Include directive does not specify which extension should be used, .ssh and .sshconfig seem to be the common extensions, when one is used. vim also names its syntax highlighting sshconfig.

Specifically, .ssh-config is NOT a common extention and ssh-config is not a common name for the syntax - at least not as far as I can tell.

As can be seen in https://github.com/github-linguist/linguist/issues/3450#issuecomment-566368051, currently only ```ssh-config is supported by GitHub.

Recommendations

I'm recommending that sshconfig, in particular, be added for syntax highlighting as it seems to be the de facto naming convention.

I'd also recommend ssh because it's the most intuitive - and I highly, highly doubt that there will be some sort of OpenSSH DSL script that would need the ssh tag in the future. That said, I do understand that, categorically, ssh might feel wrong as it would seem to describe an ssh DSL rather than a config format. However, since the layperson would reasonably assume that ssh is the syntax and probably isn't aware that sshconfig is the de facto name for the syntax.

On another note, I would argue against ssh_config as the tag as that seems to defy the tag convention, and refers specifically to the file naming convention used for substituting the / in ~/.ssh/config and the suggested system-wide configuration file name /etc/ssh/ssh_config, not the syntax itself.