feat: add completion install api

[ethanndickson]

Closes #16.

• Adds a way for API consumers to receive a recommended install path for the shell script, and to install or update the script at the path. This is done by exposing a Shell interface, which can be detected, or retrieved using the name of the shell.

• Adds a new EnumArray Flag value type, to enable use cases where multiple enum values can be supplied.
  In coder/coder this is the --output=table --column flags.
  This also makes the help formatting for these commands more consistent with Enum flags, where the possible values are stored on the type, instead of in the description. The type is represented as [Val1|Val2|...]. For reference, regular enums are Val1|Val2|....

• Fixes shell scripts not handling completions with spaces. In coder/coder column names (the argument to --column flags) often have spaces.

• Made Enum and EnumArray flag arguments now case-insensitive (using strings.EqualFold).

[ethanndickson]

This feels a bit awkward, but we need to provide it to use in the template itself, e.g. function _name, and potentially in the install path, e.g. name.fish, and it's much nicer for this function to return a Shell, instead of a string that you just have to pass back to ShellByName.

Alternatively, InstallPath and WriteCompletion could take it as an argument, but if you call one you're probably going to call the other, so I thought it'd be better to just store it.

[ethanndickson]

Kind of annoying, but we need the program name during InstallCompletion to figure out the exact header we're looking for - otherwise you obviously couldn't have multiple serpent completion scripts installed.

[ethanndickson]

These changes shouldn't really need reviewing but it's worth mentioning pflag only lets you use the shorthand with a single hyphen, so I'm getting rid of the completion for now.