Fix shell-integration-features being ignored with manual shell integration (#5048)

## Descriptions

The code was short-circuiting the shell integration setup when
`shell-integration = none`, which prevented the feature environment
variables from being set. These environment variables are needed even
for manual shell integration to work properly.

## Changes

- Extracted feature environment variables setup into a separate
`setup_features` function

- Modified the shell integration initialization to ensure features are
set up even when `shell-integration = none`

<img width="1126" alt="image"
src="https://github.com/user-attachments/assets/ceeb33f5-26ee-4a3b-a6d5-eed57848c96c"
/>


Fixes https://github.com/ghostty-org/ghostty/issues/5046

src/termio/Exec.zig

@@ -875,7 +875,11 @@ const Subprocess = struct {
             };
 
             const force: ?shell_integration.Shell = switch (cfg.shell_integration) {
-                .none => break :shell .{ null, default_shell_command },
+                .none => {
+                    // Even if shell integration is none, we still want to set up the feature env vars
+                    try shell_integration.setupFeatures(&env, cfg.shell_integration_features);
+                    break :shell .{ null, default_shell_command };
+                },
                 .detect => null,
                 .bash => .bash,
                 .elvish => .elvish,