fix: escape composed executable paths on Windows

[lorenzotumini]

Problem

#148 fixed paths containing whitespace by shell-escaping each substituted variable. On Windows, the default compiled-language commands now expand $dir/$fileNameWithoutExt into two separately quoted fragments:

"C:\path\to\dir"/"program"

When Neovim passes this through cmd.exe, the directory is interpreted as the command and the compiled executable is not launched.

Fix

Expand $dir/$fileNameWithoutExt and its backslash variant as one complete path before processing individual variables, then shell-escape that complete path.

This preserves the existing placeholder API and user configurations while retaining the whitespace fix from #148. Replacement callbacks ensure that % characters in paths remain literal.

Testing

• Full test suite under WSL/Linux: 124 passed, 0 failed
• Native Windows with Neovim 0.12.3 and cmd.exe
• End-to-end C compilation and execution on both Windows and WSL
• Directory and source filename containing spaces
• Slash and backslash command variants
• Default C, C++, and Rust command expansion