zsh: support sourcing zsh completion dynamically

Previously, you needed to save the completion script to a file and
then source it. Now, you can dynamically source completions in zsh by
running

    $ source <(rg --generate complete-zsh)

Before this commit, you would get an error after step 1.
After this commit, it should work as expected.

We also improve the FAQ item for zsh completions.

Fixes #2956

FAQ.md

@@ -94,7 +94,7 @@ Does ripgrep have support for shell auto-completion?
 
 Yes! If you installed ripgrep through a package manager on a Unix system, then
 the shell completion files included in the release archive should have been
-installed for you automatically. If not, you can generate completes using
+installed for you automatically. If not, you can generate completions using
 ripgrep's command line interface.
 
 For **bash**:
@@ -113,14 +113,31 @@ $ mkdir -p "$dir"
 $ rg --generate complete-fish > "$dir/rg.fish"
 ```
 
-For **zsh**:
+For **zsh**, the recommended approach is:
 
-```
+```zsh
 $ dir="$HOME/.zsh-complete"
 $ mkdir -p "$dir"
 $ rg --generate complete-zsh > "$dir/_rg"
 ```
 
+And then add `$HOME/.zsh-complete` to your `fpath` in, e.g., your
+`$HOME/.zshrc` file:
+
+```zsh
+fpath=($HOME/.zsh-complete $fpath)
+```
+
+Or if you'd prefer to load and generate completions at the same time, you can
+add the following to your `$HOME/.zshrc` file:
+
+```zsh
+$ source <(rg --generate complete-zsh)
+```
+
+Note though that while this approach is easier to setup, is generally slower
+than the previous method, and will add more time to loading your shell prompt.
+
 For **PowerShell**, create the completions:
 
 ```

crates/core/flags/complete/rg.zsh

@@ -434,7 +434,15 @@ _rg_types() {
   fi
 }
 
-_rg "$@"
+# Don't run the completion function when being sourced by itself.
+#
+# See https://github.com/BurntSushi/ripgrep/issues/2956
+# See https://github.com/BurntSushi/ripgrep/pull/2957
+if [[ $funcstack[1] == _rg ]] || (( ! $+functions[compdef] )); then
+  _rg "$@"
+else
+  compdef _rg rg
+fi
 
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 # ZSH COMPLETION REFERENCE