Add utility to serialize windows arguments

Add a function to convert an argv array into a single escaped string to
be passed to CreateProcess() on Windows.

Refs <https://learn.microsoft.com/en-us/cpp/c-language/parsing-c-command-line-arguments>
Refs <https://learn.microsoft.com/en-us/windows/win32/api/processthreadsapi/nf-processthreadsapi-createprocessw>

app/src/util/command.c

@@ -0,0 +1,99 @@
+#include "command.h"
+
+#include <stdlib.h>
+
+#include "util/strbuf.h"
+
+char *
+sc_command_serialize_windows(const char *const argv[]) {
+    // <https://learn.microsoft.com/en-us/cpp/c-language/parsing-c-command-line-arguments>
+    // <https://learn.microsoft.com/en-us/windows/win32/api/processthreadsapi/nf-processthreadsapi-createprocessw>
+
+    struct sc_strbuf buf;
+    bool ok = sc_strbuf_init(&buf, 1024);
+    if (!ok) {
+        return NULL;
+    }
+
+#define BUF_PUSH(C) \
+    do { \
+        ok = sc_strbuf_append_char(&buf, C); \
+        if (!ok) { \
+            goto end; \
+        } \
+    } while (0)
+
+    while (*argv) {
+        const char *arg = *argv;
+
+        BUF_PUSH('"');
+
+        // <https://learn.microsoft.com/en-us/cpp/c-language/parsing-c-command-line-arguments>
+        //
+        // """
+        // If an even number of backslashes is followed by a double quote mark,
+        // then one backslash (\) is placed in the argv array for every pair of
+        // backslashes (\\), and the double quote mark (") is interpreted as a
+        // string delimiter.
+        //
+        // If an odd number of backslashes is followed by a double quote mark,
+        // then one backslash (\) is placed in the argv array for every pair of
+        // backslashes (\\). The double quote mark is interpreted as an escape
+        // sequence by the remaining backslash, causing a literal double quote
+        // mark (") to be placed in argv.
+        // """
+        //
+        // To produce correct escaping according to what the parser will do, we
+        // must count the number of successive backslashes.
+        unsigned backslashes = 0;
+
+        for (const char *c = arg; *c; c++) {
+            switch (*c) {
+                case '"':
+                    while (backslashes) {
+                        // Double all backslashes before a quote
+                        BUF_PUSH('\\');
+                        BUF_PUSH('\\');
+                        --backslashes;
+                    }
+                    BUF_PUSH('\\');
+                    BUF_PUSH('"');
+                    backslashes = 0;
+                    break;
+                case '\\':
+                    ++backslashes;
+                    break;
+                default:
+                    while (backslashes) {
+                        // Put all backslashes as literals
+                        BUF_PUSH('\\');
+                        --backslashes;
+                    }
+                    BUF_PUSH(*c);
+                    break;
+            }
+        }
+
+        while (backslashes) {
+            // Double all backslashes before a quote
+            BUF_PUSH('\\');
+            BUF_PUSH('\\');
+            --backslashes;
+        }
+
+        BUF_PUSH('"');
+
+        ++argv;
+
+        // Argument separator
+        if (*argv) {
+            BUF_PUSH(' ');
+        }
+    }
+
+    return buf.s;
+
+end:
+    free(buf.s);
+    return NULL;
+}