Improve samples organization, add check script

samples/text/markdown.md

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+# Markdown
+
+This is an example markdown file

samples/text/plain.txt

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+This is a plain text file.

samples/text/rust.rs

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+fn main() {
+    println!("This is an example Rust source file");
+}

samples/text/x-chdr.h

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+// This is an example C header file

samples/text/x-csrc.c

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+#include <stdio.h>
+
+int main(int argc, char **argv) {
+    printf("This is an example C source file\n");
+}