Fixes 4932 postgresql default constraint functions

[griffio]

fixes #4932 - compiler error when functions are used as DEFAULT column values

😑 Currently the work-around/escape-hatch is to enclose the function in parentheses as this matches the grammar ruleLP expr RP

Add {function_expr} to the default_constraint rule

Other consideration is that ideally the function result type must be assignable to the column type 🔡 🔢 and would need to be compiler checked - currently none of the DEFAULT types are checked and can be any type of expression 🤭

CREATE TABLE t1(
  c1 UUID DEFAULT gen_random_uuid(),
  c2 INTEGER DEFAULT nextval('some_seq')
);

---

💡 Ideally - SqlDelight would provide error

CREATE TABLE t1(
  c1 INTEGER DEFAULT gen_random_uuid()
);

Postgresql does provide error
error: column "c1" is of type integer but default expression is of type uuid