jakt: Fix signature of default example's main (#5507)

In a precious version of the language, functions were defined with the
`function` keyword. They are now defined with `fn`.

Currently, the default example doen't compile.

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Co-authored-by: Rubén Rincón Blanco <git@rinconblanco.es>

examples/jakt/default.jakt

@@ -1,7 +1,7 @@
-function square(num: i32) -> i32 {
+fn square(num: i32) -> i32 {
     return num * num
 }
 
-function main() throws -> c_int {
+fn main() throws -> c_int {
     return square(num: 3) as! c_int
 }

static/modes/jakt-mode.ts

@@ -43,6 +43,7 @@ function definition(): monaco.languages.IMonarchLanguage {
             'continue',
             'throw',
             'yield',
+            'fn',
             'function',
             'extern',
             'import',