Merge 'Implement json_array' from Kacper Madej

Implements [json_array](https://sqlite.org/json1.html#jarray).
As a side quest, this PR also fixes an issue with the `CHAR` function
which didn't work properly if the parameters were  non-leaf AST nodes.
The PR is quite big, because as I mentioned in https://github.com/tursod
atabase/limbo/issues/127#issuecomment-2541307979 we had to modify
`OwnedValue::Text` to support a `subtype` parameter, which is what
SQLite does.

Closes #504

testing/json.test

@@ -55,3 +55,31 @@ do_execsql_test json5-multi-comment {
    SELECT json(' /* abc */ { /*def*/ aaa /* xyz */ : // to the end of line
           123 /* xyz */ , /* 123 */ }')
 } {{{"aaa":123}}}
+
+do_execsql_test json_array_str {
+   SELECT json_array('a')
+} {{["a"]}}
+
+do_execsql_test json_array_numbers {
+   SELECT json_array(1, 1.5)
+} {{[1,1.5]}}
+
+do_execsql_test json_array_numbers_2 {
+   SELECT json_array(1., +2., -2.)
+} {{[1.0,2.0,-2.0]}}
+
+do_execsql_test json_array_null {
+   SELECT json_array(null)
+} {{[null]}}
+
+do_execsql_test json_array_not_json {
+   SELECT json_array('{"a":1}')
+} {{["{\"a\":1}"]}}
+
+do_execsql_test json_array_json {
+   SELECT json_array(json('{"a":1}'))
+} {{[{"a":1}]}}
+
+do_execsql_test json_array_nested {
+   SELECT json_array(json_array(1,2,3), json('[1,2,3]'), '[1,2,3]')
+} {{[[1,2,3],[1,2,3],"[1,2,3]"]}}

testing/scalar-functions.test

@@ -39,6 +39,10 @@ do_execsql_test char {
   select char(108, 105)
 } {li}
 
+do_execsql_test char-nested {
+  select char(106 + 2, 105)
+} {li}
+
 do_execsql_test char-empty {
   select char()
 } {}