build: define ssize_t for MSVC in ghostty.h (#11810)

> [!WARNING]
> Review/approve this AFTER #11807 (this PR includes its commits)

92% progress with the fixes!

## Summary
- Add a conditional `ssize_t` typedef for MSVC in `include/ghostty.h`
- MSVC's `<sys/types.h>` does not define `ssize_t` (it is a POSIX type),
which causes the `translate-c` build step to fail when translating
`ghostty.h` on Windows
- Uses `SSIZE_T` from `<BaseTsd.h>`, the standard Windows SDK equivalent

## Context
The `translate-c` step translates `ghostty.h` to Zig for test
compilation. On MSVC, it fails with 3 errors on `ssize_t` (used in
`ghostty_action_move_tab_s`, `ghostty_action_search_total_s`,
`ghostty_action_search_selected_s`).

The `#ifdef _MSC_VER` guard means this only affects MSVC builds.
`BaseTsd.h` is a standard Windows SDK header and `SSIZE_T` is a signed
pointer-sized integer, matching POSIX `ssize_t` and Zig's `isize`. This
pattern is used by libuv, curl, and other cross-platform C projects.

## Test plan

### Cross-platform results (`zig build test` / `zig build
-Dapp-runtime=none test` on Windows)

| | Windows | Linux | Mac |
|---|---|---|---|
| **BEFORE** (d5aef6e84) | FAIL - 47/51 steps, 1 failed | PASS - 86/86,
2655/2678 tests, 23 skipped | PASS - 160/160, 2655/2662 tests, 7 skipped
|
| **AFTER** (a35f84db3) | FAIL - 48/51 steps, 1 failed | PASS - 86/86,
2655/2678 tests, 23 skipped | PASS - 160/160, 2655/2662 tests, 7 skipped
|

### Windows: what changed (47 -> 48 steps, translate-c fixed)

**Fixed by this PR:**
- `translate-c` - was `3 errors` (unknown type name 'ssize_t' at lines
582, 842, 847) -> `success`

**Remaining failure (pre-existing, unrelated):**
- `compile test ghostty-test` - 3 errors in libcxxabi
(`std::get_new_handler` not found, `type_info` redefinition). This is
Zig's bundled libc++ ABI conflicting with MSVC headers when compiling
the test binary. It was previously masked by the translate-c failure
blocking this step.

### Linux/macOS: no regressions
Identical pass counts and test results before and after.

## What Have I Learnt
- I tried fixing this issue the old way, googling and stuff, I
eventually figured out but it took me way more than I am prepared to
share. Yikes.

include/ghostty.h

@@ -17,6 +17,11 @@ extern "C" {
 #include <stdint.h>
 #include <sys/types.h>
 
+#ifdef _MSC_VER
+#include <BaseTsd.h>
+typedef SSIZE_T ssize_t;
+#endif
+
 //-------------------------------------------------------------------
 // Macros