refactor(managers): move regex in custom folder

[RahulGautamSingh]

Changes

• Moved regex manager in custom folder and modified the code in some test files, docs/manager.ts and manager-fingerprint generation as they depend on the file location of manager files

• Also created the file custom/index.ts and exported constants and extractPackageFile fn from it to satisfy the manager module validation in tests.

• I haven't included custom in the manager/api.ts, sincecustom is only a wrapper for custom managers and not an actual manager.

Context

Part of #19066
Tracked here: #19066 (comment)

Documentation (please check one with an [x])

• I have updated the documentation, or
• No documentation update is required

How I've tested my work (please select one)

I have verified these changes via:

• Code inspection only, or
• Newly added/modified unit tests, or
• No unit tests but ran on a real repository, or
• Both unit tests + ran on a real repository

[viceice]

did I loose something or is this just a file move refactor PR?

[RahulGautamSingh]

did I loose something or is this just a file move refactor PR?

just a file move refactor

[RahulGautamSingh]

Test run: https://github.com/RahulGautamSingh-testing/r-m/pulls

[renovate-release]

🎉 This PR is included in version 36.34.0 🎉

The release is available on:

• GitHub release
• 36.34.0

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