fix(json-mapper): prefix var to avoid var declaration that use reserved keyword in js (enum, default, etc...)

[Romakita]

Closes: #2448

[github-actions]

Benchmarks

• Machine: linux x64 | 2 vCPUs | 6.8GB Mem
• Node: v16.20.2
• Run: Wed Sep 27 2023 15:12:14 GMT+0000 (Coordinated Universal Time)
• Method: autocannon -c 100 -d 10 -p 10 localhost:3000 (two rounds; one to warm-up, one to measure)

	Version	Router	Requests/s	Latency	Throughput/Mb
fastify	3.29.4	✓	52406.4	18.65	9.34
nest-fastify	8.4.3	✓	47577.6	20.59	8.49
koa	2.13.4	✗	38674.4	25.38	6.90
fastify-injector	3.29.4	✓	26092.4	37.80	5.95
express	4.18.1	✓	9959.8	99.51	1.78
tsed-koa	7.36.3	✓	9099.8	108.91	7.95
fastify-big-json	3.29.4	✓	8798.2	112.56	101.22
nest	8.4.3	✓	8767.4	113.29	2.11
express-injector	4.18.1	✗	7946.2	124.74	1.81
tsed-express	7.36.3	✓	7386.4	134.14	1.35
express-morgan	4.18.1	✗	5806.8	170.56	1.04

Explanation

The benchmark shows a performance difference between the frameworks. We note that Ts.ED is often last. In fact, Ts.ED uses features useful to a production application which reduce its performance.

For example, Ts.ED initializes a sandbox (async_hook) for each request in order to work in an isolated context if necessary.
It also initializes the elements necessary for monitoring requests in a log manager.

All this at a necessary cost that reflects the reality of a production application ;)

[Romakita]

🎉 This PR is included in version 7.36.4 🎉

The release is available on:

• v7.36.4
• GitHub release

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