Transaction types

Nadie ha tomado este issue todavía.

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).

Comienza con lib/transaction.js y compara su gestión de transacciones con la documentación de SQLite sobre BEGIN ... TRANSACTION. Comprueba cómo gestiona el conector DEFERRED, IMMEDIATE y EXCLUSIVE, y qué ocurre con los valores de nivel de aislamiento de LoopBack. El trabajo estará terminado cuando las opciones de transacción compatibles con SQLite se gestionen correctamente sin usar nombres de nivel de aislamiento no compatibles.

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