[bc/en] New tutorial (#3519)

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+---
+language: bc
+contributors:
+    - ["Btup"]
+filename: learnbc.bc
+---
+```c
+/*This is a multi-
+line comment.*/
+# This is also a (one-line) comment! (in GNU bc).
+
+    /*1. Variables and control structures*/
+num = 45 /*All variables save only doubles, and you cannot save
+    string constants directly.*/
+num = 45; /*You can choose to add a semicolon after
+    every statement. This is optional.*/
+/*Blocks are denoted using the {} operators(similar to C):*/
+while(num < 50) {
+    num += 1 /*equivalent to num=num+1.
+    a = a op b is equivalent to a op= b.*/
+}
+/*And there are ++(increment) and --(decrement) operators.*/
+/*There are 3 special variables:
+scale: defines the scale of the double numbers.
+ibase: defines the base of input.
+obase: defines the base of output.*/
+/*If clauses:*/
+hour = read() /*Input a number*/
+
+if(hour < 12) { /*Operators are exactly like C.*/
+    print "Good morning\n" /*"print" outputs strings or variables
+	separated by commas.*/
+} else if(hour == 12) {
+    print "Hello\n"
+    /*Escaping sequences start with a \ in a string.
+    In order to make the escaping sequences clearer, here
+	is a simplified list of them that will work in bc:
+    \b: backspace
+    \c: carriage return
+    \n: newline
+    \t: tab
+    \\: backslash*/
+} else {
+    /*Variables are global by default.*/
+    thisIsGlobal = 5
+    /*You can make a variable local. Use the "auto" keyword in a function.*/
+}
+
+/*Every variable is pre-set to 0.*/
+num = blankVariable /*num is set to 0.*/
+
+/*Like C, only 0 is falsy.*/
+if(!num) {print "false\n"}
+
+/*Unlike C, bc does not have the ?: operators. For example,
+ this block of code will cause an error:
+a = (num) ? 1 : 0
+However, you can simulate one:*/
+a = (num) && (1) || (0) /*&& is and, || is or*/
+
+/*For loops*/
+num = 0
+for(i = 1; i <= 100; i++) {/*Similar to the C for loop.*/
+    num += i
+}
+
+    /*2.Functions and Arrays*/
+define fac(n) { /*define a function using define.*/
+    if(n == 1 || n == 0) {
+        return 1 /*return a value*/
+    }
+    return n * fac(n - 1) /*recursion is possible*/
+}
+
+/*Closures and anonymous functions are impossible.*/
+
+num = fac(4) /*24*/
+
+/*This is an example of local variables:*/
+define x(n) {
+    auto x
+    x = 1
+    return n + x
+}
+x(3) /*4*/
+print x /*It turns out that x is not accessible out of the function.*/
+/*Arrays are equivalent to the C array.*/
+for(i = 0; i <= 3; i++) {
+    a[i] = 1
+}
+/*Access it like this:*/
+print a[0], " ", a[1], " ", a[2], " ", a[3], "\n"
+quit /*Add this line of code to make sure
+that your program exits. This line of code is optional.*/
+```
+Enjoy this simple calculator! (Or this programming language, to be exact.)
+
+This whole program is written in GNU bc. To run it, use ```bc learnbc.bc```.