Preserve ZSH options in the shell integration (#2950)

This fixes the #2847. There were two problems:
1. Documentation made it easy to use the bash example as a template for
calling other integrations
2. f the ZSH integration script is called the same way as for bash, it
would clobber the options

This PR fixes both. Now the ZSH script can be simply called with
`source`.

Here is why the options were clobbered, and how the method of calling
affected it:

The `-L` flag (local scope) ensures that the emulation mode only applies
to the current function scope. When the function ends, the shell reverts
to its previous state. When called outside of a function, the flags
persist even after the script execution ends.

The recommended way of calling the ZSH integration included `autoload
-Uz` that created a function with the integration script as its body.
When called directly, the assumptions about being in the function body
broke.

This PR moves a lot of code into a function, so it's best to review with
whitespace ignored in the diff.

README.md

@@ -286,13 +286,16 @@ if [ -n "${GHOSTTY_RESOURCES_DIR}" ]; then
 fi
 ```
 
+For details see <a href="https://github.com/ghostty-org/ghostty/blob/main/src/shell-integration/README.md">shell-integration/README.md</a>.
+
 Each shell integration's installation instructions are documented inline:
 
 | Shell    | Integration                                                                                    |
-| ------ | ---------------------------------------------------------------------------------------------- |
+| -------- | ---------------------------------------------------------------------------------------------- |
 | `bash`   | `${GHOSTTY_RESOURCES_DIR}/shell-integration/bash/ghostty.bash`                                 |
 | `fish`   | `${GHOSTTY_RESOURCES_DIR}/shell-integration/fish/vendor_conf.d/ghostty-shell-integration.fish` |
 | `zsh`    | `${GHOSTTY_RESOURCES_DIR}/shell-integration/zsh/ghostty-integration`                           |
+| `elvish` | `${GHOSTTY_RESOURCES_DIR}/shell-integration/elvish/lib/ghostty-integration.elv`                |
 
 ### Terminfo
 

src/shell-integration/README.md

@@ -24,6 +24,13 @@ must be explicitly enabled (`shell-integration = bash`).
 Bash shell integration can also be sourced manually from `bash/ghostty.bash`.
 This also works for older versions of Bash.
 
+```bash
+# Ghostty shell integration for Bash. This must be at the top of your bashrc!
+if [ -n "${GHOSTTY_RESOURCES_DIR}" ]; then
+    builtin source "${GHOSTTY_RESOURCES_DIR}/shell-integration/bash/ghostty.bash"
+fi
+```
+
 ### Elvish
 
 For [Elvish](https://elv.sh), `$GHOSTTY_RESOURCES_DIR/src/shell-integration`
@@ -59,3 +66,9 @@ For `zsh`, Ghostty sets `ZDOTDIR` so that it loads our configuration
 from the `zsh` directory. The existing `ZDOTDIR` is retained so that
 after loading the Ghostty shell integration the normal Zsh loading
 sequence occurs.
+
+```bash
+if [[ -n $GHOSTTY_RESOURCES_DIR ]]; then
+  "$GHOSTTY_RESOURCES_DIR"/shell-integration/zsh/ghostty-integration
+fi
+```

src/shell-integration/zsh/ghostty-integration

@@ -25,9 +25,7 @@
 # Ghostty in all shells should add the following lines to their .zshrc:
 #
 #   if [[ -n $GHOSTTY_RESOURCES_DIR ]]; then
-#     autoload -Uz -- "$GHOSTTY_RESOURCES_DIR"/shell-integration/zsh/ghostty-integration
-#     ghostty-integration
-#     unfunction ghostty-integration
+#     "$GHOSTTY_RESOURCES_DIR"/shell-integration/zsh/ghostty-integration
 #   fi
 #
 # Implementation note: We can assume that alias expansion is disabled in this
@@ -35,6 +33,9 @@
 # builtins with `builtin` to avoid accidentally invoking user-defined functions.
 # We avoid `function` reserved word as an additional defensive measure.
 
+# Note that updating options with `builtin emulate -L zsh` affects the global options
+# if it's called outside of a function. So nearly all code has to be in functions.
+_entrypoint() {
     builtin emulate -L zsh -o no_warn_create_global -o no_aliases
 
     [[ -o interactive ]]              || builtin return 0  # non-interactive shell
@@ -78,6 +79,7 @@ typeset -gi _ghostty_fd
     # the integration.
     builtin typeset -ag precmd_functions
     precmd_functions+=(_ghostty_deferred_init)
+}
 
 _ghostty_deferred_init() {
     builtin emulate -L zsh -o no_warn_create_global -o no_aliases
@@ -310,3 +312,5 @@ _ghostty_deferred_init() {
     # to unfunction themselves when invoked. Unfunctioning is done by calling code.
     builtin unfunction _ghostty_deferred_init
 }
+
+_entrypoint