feat: enhance build configuration with C++ standard support and variable substitution

- Added `cxx_standard` field to the build configuration for specifying C++ standards.
- Updated documentation to reflect new `cxx_standard` and variable substitution capabilities in `cflags`, `ldflags`, and `include`.
- Implemented logic to handle C++ standard flags in the build process.
- Modified object file naming to include source file extensions.
- Updated various builder modules to support new language parsing logic.
- Introduced issue and feature request templates for better community engagement.

src/core/config.rs

@@ -100,6 +100,8 @@ pub struct PackageConfig {
 pub struct BuildConfig {
     pub language: LanguageConfig,
     pub standard: String,
+    #[serde(default)]
+    pub cxx_standard: Option<String>,
     pub compiler: String,
     #[serde(default)]
     pub kind: Option<String>,