Transaction types

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).

Start with lib/transaction.js and compare its transaction handling with SQLite's BEGIN ... TRANSACTION documentation. Check how the connector handles DEFERRED, IMMEDIATE, and EXCLUSIVE and what happens for LoopBack's isolation-level values. The work is done when SQLite-supported transaction options are handled correctly without using unsupported isolation-level names.

Written by the indexing model from the issue text.