refactor(common,core): use shared utils whenever possible

[micalevisk]

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?

In several places, utilities functions declared at https://github.com/nestjs/nest/blob/939f2d56d1c9ec25108ada7053dfc51cbc6be7c2/packages/common/utils/shared.utils.ts are not used when they could be used

What is the new behavior?

Every typeof x === y and typeof x !== y was replaced with some method from @nestjs/common/utils/shared.utils.ts file

The only line I didn't change was the following:

nest/packages/core/router/router-module.ts

Line 45 in 259b6fb

if (typeof routeOrType === 'function') {

for whatever reason, using isFunction(routeOrType) introduces a compile error:

packages/core/router/router-module.ts(49,23): error TS2339: Property 'children' does not exist on type 'Type<any> | RouteTree'.
  Property 'children' does not exist on type 'Type<any>'.
  
packages/core/router/router-module.ts(52,54): error TS2339: Property 'children' does not exist on type 'Type<any> | RouteTree'.
  Property 'children' does not exist on type 'Type<any>'.

Does this PR introduce a breaking change?

• Yes
• No

Other information

I choose to didn' t touch any .spec.ts file, and only files under packages directory.

[coveralls]

Pull Request Test Coverage Report for Build 640b0495-a3cd-41ce-81e5-19830578f43a

• 23 of 23 (100.0%) changed or added relevant lines in 13 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.007%) to 94.087%

---

Totals
Change from base Build 41b0d386-1506-44ca-bc4f-822370edf048:	0.007%
Covered Lines:	5665
Relevant Lines:	6021

---

💛 - Coveralls

[kamilmysliwiec]

LGTM