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).

Beginnen Sie mit lib/transaction.js und vergleichen Sie die Transaktionsverarbeitung dort mit der Dokumentation von SQLite zu BEGIN ... TRANSACTION. Prüfen Sie, wie der Connector DEFERRED, IMMEDIATE und EXCLUSIVE behandelt und was bei den Isolationsstufenwerten von LoopBack passiert. Die Arbeit ist abgeschlossen, wenn von SQLite unterstützte Transaktionsoptionen korrekt behandelt werden, ohne nicht unterstützte Namen für Isolationsstufen zu verwenden.

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