Proposal for the new `slicec` CLI

[InsertCreativityHere]

Now that slicec is a binary which runs plugins vs a library used for other compilers, we need to augment it's CLI.
This is an example of the current proposal:

slicec \
    --generator /path/to/icerpc-gen.bat \
    --generator /path/to/zeroc-gen.bat \
    --output-dir ~/code \
    Test1.slice Test2.slice ...

It would also support the usual -- to distinguish between the Slice file inputs vs the command-line options.

There are no names to the generators (unlike protoc which uses go_out=...), and there is a single output-dir option, shared by all generators. The thinking is that it's going to be most common than you'll be compiling for a single language at a time, and want all of you generated code in the same place.
It'd be tedious to specify the same output directory for both your .cs and IceRpc.cs files.

If you want your generated code to end up in multiple output directories, you need to call slicec multiple times.

---

This isn't going to be relevant for us anytime soon, but we also support passing arbitrary options to each generator with the syntax:

--generator /my/generator.exe;key1=value1;key2=value2

Also, of course, we'll want to support quoted things too, and escaping for ;, =, and ".

--generator "/my/generator.exe;key1=value1;key2=value2"

---

We'd support -G as a shorthand for --generator and -O as a shorthand for --output-dir.

Note that slicec already has a handful of command-line options, which don't conflict with this proposal.
-D to define preprocessor symbols, -R for reference files, --dry-run to run but not generate code etc.

[InsertCreativityHere]

After some discussion, we've decided on the following changes:

Use , instead of ; as our separator

; has a special meaning on the command line (separating commands), and in the C# tooling we use. So it'd be nicer to avoid using it as our separator. The next best separator that we all like is ,. No special connotation, and is universally understood to be a list separator already.

Only Escape Necessary Characters

Right now, we allow you to escape any character by placing a backslash in front of it, \= -> =, \\ -> \, \a=a.
This is a simple rule, but causes headaches for Windows users, where \ is a common path character. For example:
C:\Users\Me\generator would have to be written C:\\Users\\Me\\generator. Painful.

Instead, we should only support escaping the bare minimum number of characters, namely: \, -> , and \= -> =.
And nothing else. If a \ appears in any other context, we treat it as a normal, literal, backslash.

[InsertCreativityHere]

The generator CLI was implemented in #752, and refined in #767