Transaction types

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Hi! 👋

Firstly, thanks for your work on this project! 🙂

Today I used [patch-package](https://github.com/ds300/patch-package) to patch `loopback-connector-sqlite3@3.0.0` for the project I'm working on.

The issue i'm facing is that loopback uses enum of isolation levels (below), which doesn't correspond to the sqlite3-supported list of transaction options.

The Loopback enum:
```
export declare enum IsolationLevel {
READ_COMMITTED = "READ COMMITTED",
READ_UNCOMMITTED = "READ UNCOMMITTED",
SERIALIZABLE = "SERIALIZABLE",
REPEATABLE_READ = "REPEATABLE READ"
}
```
Supported SQLITE3 `BEGIN ... TRANSACTION` is DEFERRED, IMMEDIATE and EXCLUSIVE (see https://www.sqlite.org/lang_transaction.html)

Here is the diff that solved my problem:

```diff
diff --git a/node_modules/loopback-connector-sqlite3/lib/transaction.js b/node_modules/loopback-connector-sqlite3/lib/transaction.js
index 580cc28..c348094 100644
--- a/node_modules/loopback-connector-sqlite3/lib/transaction.js
+++ b/node_modules/loopback-connector-sqlite3/lib/transaction.js
@@ -22,6 +24,9 @@ function mixinTransaction(SQLite3) {
cb = isolationLevel;
isolationLevel = 'DEFERRED';
}
+ if (!['DEFERRED', 'IMMEDIATE', 'EXCLUSIVE'].includes(isolationLevel)) {
+ isolationLevel = 'DEFERRED';
+ }

debug('Begin a transaction with isolation level: %s', isolationLevel);
this._getConnection(function(err, connection) {
```

This issue body was [partially generated by patch-package](https://github.com/ds300/patch-package/issues/296).

Commencez par lib/transaction.js et comparez sa gestion des transactions avec la documentation de SQLite concernant BEGIN ... TRANSACTION. Vérifiez comment le connecteur gère DEFERRED, IMMEDIATE et EXCLUSIVE, ainsi que ce qui se passe pour les valeurs de niveau d’isolation de LoopBack. Le travail est terminé lorsque les options de transaction prises en charge par SQLite sont gérées correctement sans utiliser de noms de niveau d’isolation non pris en charge.

Rédigé par le modèle d'indexation à partir du texte de l'issue.