[LIGO-47] Allow let .. in in lambda body

Problem: In multisig.mligo, else clause on line 36 ends on line 55,
while it should end on line 80. This happens because of incorrect
lambda parsing on lines 42-43. In particular, `let valid, keys = vk`
in lambda body gets parsed as a
`tuple (fun_app "let" "valid") (binary_op "=" "keys" "vk")`, and `in`
is considered as a part of the outer `let`.

Solution: Allow `_program` instead of `_expr` as the lambda body.

tools/lsp/squirrel/grammar/camligo/grammar.js

@@ -229,7 +229,7 @@ module.exports = grammar({
       "fun",
       repeat1(field("arg", $._paren_pattern)),
       "->",
-      field("body", $._expr)
+      field("body", $._program)
     ),
 
     list_expr: $ => seq(