[WIP] Fix parse failures of COLLATE operator

[Copilot]

Thanks for assigning this issue to me. I'm starting to work on it and will keep this PR's description up to date as I form a plan and make progress.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Parse failures of COLLATE:</issue_title>
<issue_description>We can use COLLATE to change collating sequence for a single expression:

The COLLATE operator is a unary postfix operator that assigns a collating sequence to an expression. The collating sequence set by the COLLATE operator overrides the collating sequence determined by the COLLATE clause in a table column definition. See the detailed discussion on collating sequences in the Datatype In SQLite3 document for additional information.

https://www.sqlite.org/lang_expr.html

sqlite> SELECT * FROM vals;
+---+
| A |
+---+
| a |
| b |
| c |
| d |
| e |
+---+
sqlite> SELECT * FROM vals WHERE a < 'C' ;
sqlite> SELECT * FROM vals WHERE a < 'C' COLLATE NOCASE;
+---+
| A |
+---+
| a |
| b |
+---+

However, such queries produce the following parse error: https://go.dev/play/p/q1rykRy4OZ7

package main

import (
	"fmt"
	"strings"

	"github.com/rqlite/sql"
)

func main() {
	for _, q := range []string{
		`SELECT * FROM vals WHERE a < 'C' COLLATE NOCASE`,
	} {
		p := sql.NewParser(strings.NewReader(q))
		s, err := p.ParseStatement()
		fmt.Printf("%q\n   err: %v\n  stmt: %#v\n", q, err, s)
	}
}

produces:

"SELECT * FROM vals WHERE a < 'C' COLLATE NOCASE"
   err: 1:34: expected semicolon or EOF, found 'COLLATE'
  stmt: &sql.SelectStatement{WithClause:(*sql.WithClause)(nil), Values:sql.Pos{Offset:0, Line:0, Column:0}, ValueLists:[]*sql.ExprList(nil), Select:sql.Pos{Offset:0, Line:1, Column:1}, Distinct:sql.Pos{Offset:0, Line:0, Column:0}, All:sql.Pos{Offset:0, Line:0, Column:0}, Columns:[]*sql.ResultColumn{(*sql.ResultColumn)(0xc000146000)}, From:sql.Pos{Offset:9, Line:1, Column:10}, Source:(*sql.QualifiedTableName)(0xc000148000), Where:sql.Pos{Offset:19, Line:1, Column:20}, WhereExpr:(*sql.BinaryExpr)(0xc000104080), Group:sql.Pos{Offset:0, Line:0, Column:0}, GroupBy:sql.Pos{Offset:0, Line:0, Column:0}, GroupByExprs:[]sql.Expr(nil), Having:sql.Pos{Offset:0, Line:0, Column:0}, HavingExpr:sql.Expr(nil), Window:sql.Pos{Offset:0, Line:0, Column:0}, Windows:[]*sql.Window(nil), Union:sql.Pos{Offset:0, Line:0, Column:0}, UnionAll:sql.Pos{Offset:0, Line:0, Column:0}, Intersect:sql.Pos{Offset:0, Line:0, Column:0}, Except:sql.Pos{Offset:0, Line:0, Column:0}, Compound:(*sql.SelectStatement)(nil), Order:sql.Pos{Offset:0, Line:0, Column:0}, OrderBy:sql.Pos{Offset:0, Line:0, Column:0}, OrderingTerms:[]*sql.OrderingTerm(nil), Limit:sql.Pos{Offset:0, Line:0, Column:0}, LimitExpr:sql.Expr(nil), Offset:sql.Pos{Offset:0, Line:0, Column:0}, OffsetComma:sql.Pos{Offset:0, Line:0, Column:0}, OffsetExpr:sql.Expr(nil)}
```</issue_description>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes Parse failures of COLLATE: #95

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