fix(postgres): escape array literal values containing backslash

[Strengthless]

Fixes #4796.

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Files	Coverage Δ
packages/postgresql/src/PostgreSqlPlatform.ts	99.21% <100.00%> (ø)

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[Strengthless]

Not sure whether the new title is more descriptive or misleading...

The issue seems to be some weird bug specific to single backslash, as both orm.em.create(User, { options: ['\\\\'] }) and orm.em.create(User, { options: ['\\test'] }) worked perfectly when I was trying to debug my code, if I remember correctly. Might want to double check though.

But I guess it doesn't really matter ¯\_(ツ)_/¯