Add support for better-sqlite3 driver

[michaelbromley]

Is your feature request related to a problem? Please describe.
It appears that using the sqlite3 package leads to concurrency issues when creating products in the Admin UI. Creating a product with more than 1 variant when using the sqlite driver in the TypeORM connection options results in:

"Transaction already started for the given connection, commit current transaction before starting a new one."

This issue is explored here: typeorm/typeorm#1884

Describe the solution you'd like
I tried switching over to the better-sqlite3 package (support for this was added in TypeORM 0.2.26) and the issue goes away.

The problem is that currently the DefaultSearchPlugin will fail because it does not recognise the 'better-sqlite3' type:

vendure/packages/core/src/plugin/default-search-plugin/fulltext-search.service.ts

Lines 87 to 103 in 5d0c6ae

	private setSearchStrategy() {
	switch (this.connection.rawConnection.options.type) {
	case 'mysql':
	case 'mariadb':
	this.searchStrategy = new MysqlSearchStrategy(this.connection);
	break;
	case 'sqlite':
	case 'sqljs':
	this.searchStrategy = new SqliteSearchStrategy(this.connection);
	break;
	case 'postgres':
	this.searchStrategy = new PostgresSearchStrategy(this.connection);
	break;
	default:
	throw new InternalServerError(`error.database-not-supported-by-default-search-plugin`);
	}
	}

The solution is to:

1. Add better-sqlite3 to the above switch statement.
2. Use better-sqlite3 by default in @vendure/create