Refactor build flags handling and add tool alias management

- Introduced a new method `apply_default_string` to streamline the application of default values for string fields in build flags.
- Enhanced `apply_flags_table` to utilize the new method for setting various build flags, improving code readability and maintainability.
- Added support for additional build flags: `fuse_ld`, `ranlib`, `strip`, `nm`, `objcopy`, `objdump`, and `readelf`.
- Implemented a new command `set` to manage tool aliases for compilers, linkers, and shells, allowing users to set preferred tools in the build environment.
- Improved error handling and validation for tool alias configuration, ensuring that existing non-symlink files are not replaced.
- Added tests to verify the correct behavior of tool alias management and build flag application.

contrib/user.depot.toml.example

@@ -10,8 +10,11 @@
 prefix = "/usr/local"
 
 # Personal toolchain preference.
+# tool_dir = "/usr/bin"
 cc = "clang"
-# cxx = "clang++"
+# cxx = "$TOOL_DIR/clang++"
+# ranlib = "$TOOL_DIR/ranlib"
+# strip = "$TOOL_DIR/strip"
 
 [build.flags]
 # User-local compiler preferences.
@@ -22,6 +25,7 @@ carch = "x86_64"
 # Examples:
 # cxxflags = ["-O2", "-g"]
 # ldflags = ["-Wl,-O1"]
+# fuse_ld = "lld"
 # makeflags = "-j4"
 
 # Optional install directory overrides for supported builders.