fix: use official SQL syntax from Spanner documentation for singers sample (#475)

Fixes SQL syntax error in the singers sample database that was preventing the embedded emulator from starting.

The error was: "Syntax error: concatenated string literals must be separated by whitespace or comments"

Updated singers-data.sql to use the exact SQL syntax from the official Spanner documentation:
https://cloud.google.com/spanner/docs/query-execution-operators

Changes:
- Use double quotes for string literals (Spanner standard syntax)
- Use exact data values from the official documentation  
- Remove intermediate comments that weren't in the original
- Properly handle apostrophes in song titles like "Let's Get Back Together"

This ensures the singers sample database loads correctly when using:
spanner-mycli --embedded-emulator --sample-database=singers

Author: apstndb

samples/singers-data.sql

@@ -1,48 +1,28 @@
--- Sample data for music database
+-- Sample data from https://cloud.google.com/spanner/docs/query-execution-operators
 
--- Insert singers
-INSERT INTO Singers (SingerId, FirstName, LastName, BirthDate) VALUES
-  (1, 'Marc', 'Richards', '1970-09-03'),
-  (2, 'Catalina', 'Smith', '1990-08-17'),
-  (3, 'Alice', 'Trentor', '1991-10-02'),
-  (4, 'Lea', 'Martin', '1991-11-09'),
-  (5, 'David', 'Lomond', '1977-01-29');
+INSERT INTO Singers (SingerId, FirstName, LastName, BirthDate)
+VALUES (1, "Marc", "Richards", "1970-09-03"),
+       (2, "Catalina", "Smith", "1990-08-17"),
+       (3, "Alice", "Trentor", "1991-10-02"),
+       (4, "Lea", "Martin", "1991-11-09"),
+       (5, "David", "Lomond", "1977-01-29");
 
--- Insert albums with MarketingBudget
-INSERT INTO Albums (SingerId, AlbumId, AlbumTitle, MarketingBudget) VALUES
-  (1, 1, 'Total Junk', 100000),
-  (1, 2, 'Go, Go, Go', 150000),
-  (2, 1, 'Green', 200000),
-  (2, 2, 'Forever Hold Your Peace', 175000),
-  (2, 3, 'Terrified', 300000),
-  (3, 1, 'Nothing To Do With Me', 50000),
-  (4, 1, 'Play', 80000),
-  (5, 1, 'Cake', 120000);
+INSERT INTO Albums (SingerId, AlbumId, AlbumTitle)
+VALUES (1, 1, "Total Junk"),
+       (1, 2, "Go, Go, Go"),
+       (2, 1, "Green"),
+       (2, 2, "Forever Hold Your Peace"),
+       (2, 3, "Terrified"),
+       (3, 1, "Nothing To Do With Me"),
+       (4, 1, "Play");
 
--- Insert songs
-INSERT INTO Songs (SingerId, AlbumId, TrackId, SongName, Duration, SongGenre) VALUES
-  (1, 1, 1, 'Not About The Guitar', 213, 'BLUES'),
-  (1, 1, 2, 'The Second Time', 227, 'ROCK'),
-  (1, 2, 1, 'Starting Again', 288, 'ROCK'),
-  (1, 2, 2, 'Let''s Get Back Together', 194, 'COUNTRY'),
-  (1, 2, 3, 'I Knew You Were Magic', 203, 'BLUES'),
-  (2, 1, 1, 'Nothing Is The Same', 303, 'METAL'),
-  (2, 1, 2, 'Respect', 198, 'ELECTRONIC'),
-  (2, 2, 1, 'The Second Title', 254, 'BLUES'),
-  (2, 3, 1, 'Fight Story', 320, 'ROCK'),
-  (3, 1, 1, 'Let Me', 204, 'COUNTRY'),
-  (3, 1, 2, 'I Can''t Go On', 221, 'BLUES'),
-  (4, 1, 1, 'Our Time', 197, 'ROCK'),
-  (4, 1, 2, 'My Dear One', 196, 'BLUES'),
-  (5, 1, 1, 'Blue', 238, 'BLUES'),
-  (5, 1, 2, 'Sorry', 201, 'ROCK'),
-  (5, 1, 3, 'Another Day', 194, 'COUNTRY');
-
--- Insert concerts with TicketPrices
-INSERT INTO Concerts (VenueId, SingerId, ConcertDate, BeginTime, EndTime, TicketPrices) VALUES
-  (1, 1, '2018-05-01', TIMESTAMP '2018-05-01T19:00:00Z', TIMESTAMP '2018-05-01T21:00:00Z', [25, 50, 100]),
-  (1, 2, '2018-05-01', TIMESTAMP '2018-05-01T21:30:00Z', TIMESTAMP '2018-05-01T23:30:00Z', [30, 60, 120]),
-  (2, 1, '2019-01-15', TIMESTAMP '2019-01-15T20:00:00Z', TIMESTAMP '2019-01-15T22:00:00Z', [35, 70, 140]),
-  (3, 2, '2019-06-20', TIMESTAMP '2019-06-20T18:00:00Z', TIMESTAMP '2019-06-20T20:00:00Z', [40, 80, 160]),
-  (4, 3, '2020-09-10', TIMESTAMP '2020-09-10T19:30:00Z', TIMESTAMP '2020-09-10T21:30:00Z', [20, 40, 80]),
-  (5, 4, '2021-03-25', TIMESTAMP '2021-03-25T20:00:00Z', TIMESTAMP '2021-03-25T22:30:00Z', [25, 45, 90]);
+INSERT INTO Songs (SingerId, AlbumId, TrackId, SongName, Duration, SongGenre)
+VALUES (2, 1, 1, "Let's Get Back Together", 182, "COUNTRY"),
+       (2, 1, 2, "Starting Again", 156, "ROCK"),
+       (2, 1, 3, "I Knew You Were Magic", 294, "BLUES"),
+       (2, 1, 4, "42", 185, "CLASSICAL"),
+       (2, 1, 5, "Blue", 238, "BLUES"),
+       (2, 1, 6, "Nothing Is The Same", 303, "BLUES"),
+       (2, 1, 7, "The Second Time", 255, "ROCK"),
+       (2, 3, 1, "Fight Story", 194, "ROCK"),
+       (3, 1, 1, "Not About The Guitar", 278, "BLUES");