chore: replace lodash with es-toolkit

[hwibaski]

Replace lodash utility functions with es-toolkit equivalents to improve performance. Update imports in config service and merge-configs utility.

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior?

The config service and merge-configs utility are currently using lodash utility functions, which increases the bundle size and introduces unnecessary dependencies.

Issue Number: N/A

What is the new behavior?

Replaced lodash utility functions with equivalent es-toolkit functions to reduce bundle size and improve performance. Also updated import statements in config service and merge-configs utility accordingly.

Does this PR introduce a breaking change?

• Yes
• No

Other information

• The es-toolkit/compat module fully covers the existing lodash test coverage, ensuring no side effects or regressions.
• Lodash has not had a major release since v4 on January 13, 2016 (lodash releases).
• Lodash is no longer actively maintained, evidenced by unanswered user questions regarding a new version (Any new version release? lodash/lodash#5989).
• The latest commit to the planned v5 of Lodash was 8 months ago (v5-wip branch).
• @types/lodash contains numerous any types, negatively impacting overall application type safety.