parser: prepare for better VLS integration, more accurate parser errors (#7119)

cmd/v/help/build.txt

@@ -124,6 +124,9 @@ The build flags are shared by the build and run commands:
 
    -W
       Treat all warnings as errors, even in development builds.
+   
+   -Wfatal-errors
+      Unconditionally exit with exit(1) after the first error. Useful for scripts/tooling that calls V.
 
 For C-specific build flags, use `v help build-c`.