doc: add info about _ separator in literals (#5823)

doc/docs.md

@@ -388,18 +388,22 @@ println(s) // "hello\nworld"
 ```v
 a := 123
 ```
+
 This will assign the value of 123 to `a`. By default `a` will have the
 type `int`.
 
 You can also use hexadecimal notation for integer literals:
+
 ```v
 a := 0x7B
 ```
 ... or binary notation for integer literals:
+
 ```v
 a := 0b01111011
 ```
 ... or octal notation for specifying integer literals:
+
 ```v
 a := 0o173
 ```
@@ -407,8 +411,18 @@ a := 0o173
 All of these will assign the same value 123 to `a`. `a` will have the
 type `int` no matter what notation you have used for the integer literal.
 
+V also supports writing numbers with `_` as separator:
+
+```v
+num := 1_000_000 // same as 1000000
+three := 0b0_11 // same as 0b11
+float_num := 3_122.55  // same as 3122.55
+hexa := 0xF_F // same as 255
+oct := 0o17_3 // same as 0o173
+```
 
 If you want a different type of integer, you can use casting:
+
 ```v
 a := i64(123)
 b := byte(42)
@@ -416,6 +430,7 @@ c := i16(12345)
 ```
 
 Assigning floating point numbers works the same way:
+
 ```v
 f := 1.0
 f1 := f64(3.14)

vlib/builtin/int_test.v

@@ -144,6 +144,33 @@ fn test_oct() {
 	assert x9 == 0
 }
 
+fn test_num_separator() {
+	// int
+	assert 100_000_0 == 1000000
+	assert -2_23_4_6 == -22346
+	assert 230_ == 230
+
+	// bin
+	assert 0b0_11 == 3
+	assert -0b0_100 == -4
+	assert 0b010_ == 2
+
+	// oct
+	assert 0o_173 == 123
+	assert -0o_175 == -125
+	assert -0o175_ == -125
+
+	// hex
+	assert 0x_FF == 255
+	assert 0xFF_ == 255
+	assert 0xF_F == 255
+
+	// f32 or f64
+	assert 312_2.55 == 3122.55
+	assert 312_2.55 == 3122.55
+	
+}
+
 fn test_int_decl() {
 	x1 := 0
 	x2 := 1333