fix(sqlite): Fix UNIQUE parsing

[VaggelisD]

Fixes #4291

The following issues appear throughout these cases are the following:

1. UNIQUE can be used as a standalone column constraint:

CREATE TABLE foo (bar TEXT UNIQUE REFERENCES "baz")

However, parser.py::_parse_unique() can end up consuming the following constraints, so this PR overrides it to add the appropriate lookahead.

2. Implicit aliases can be used in column definitions, e.g:

CREATE TABLE foo (bar baz TEXT) -> fine
CREATE TABLE foo (bar AS baz TEXT) -> error

Which is solved by overriding parser.py::_parse_field_def() to also parse the alias & generating it back without the AS prefix.

[VaggelisD]

It's actually not clear what's the behavior leading to the 2nd issue; If it was an alias, the AS version wouldn't lead to an error and it would be possible to select it directly. Here are some interesting examples of that:

• It's valid to include multiple identifiers before the type

sqlite> create table test (foo bar baz TEXT); 
sqlite> insert into test values (1);
sqlite> select * from test;
1
sqlite> select foo from test;
1

• Upon inspecting the table definition, the 2nd+ identifier(s) leak into the type (?)

sqlite> PRAGMA table_info("test");
0|foo|bar baz TEXT|0||0

For these reasons, I'll revert the solution to (2)