Improve new CLI UX by supporting short -E flag for --expr

Change-Id: I55881c846da8416a92a14deedfa5bbbf09a122fb

src/libcmd/installables.cc

@@ -187,6 +187,7 @@ SourceExprCommand::SourceExprCommand()
 
     addFlag({
         .longName = "expr",
+        .shortName = 'E',
         .description = "Interpret [*installables*](@docroot@/command-ref/new-cli/nix.md#installables) as attribute paths relative to the Nix expression *expr*.",
         .category = installablesCategory,
         .labels = {"expr"},

src/nix/eval.md

@@ -8,10 +8,10 @@ R""(
   # nix eval --expr '1 + 2'
   ```
 
-* Evaluate a Nix expression to JSON:
+* Evaluate a Nix expression to JSON using the short-form expression flag:
 
   ```console
-  # nix eval --json --expr '{ x = 1; }'
+  # nix eval --json -E '{ x = 1; }'
   {"x":1}
   ```