[typescript][axios] Handle boolean enum correctly

Created by: haedaal

closes #9024 (closed)

with following schema

    "schemas": {
      "BooleanEnum": {
        "enum": ["true", "false"],
        "type": "boolean"
      },

https://gist.github.com/haedaal/a771d6f3df337b624ae8a2c2d176c0db

typescript-fetch currently generates

export enum BooleanEnum {
    True = 'true',
    False = 'false'
}

with this pr

export type BooleanEnum = true | false

by using 'type' rather than 'enum' we breaks consistency, but TS18033 does not allow this

export enum BooleanEnum {
    True = true, // Only numeric enums can have computed members, but this expression has type 'true' ...
    False = false
}

this pr has 2 changes.

1. by adding isBoolean, generate model correctly
2. fix typescript-axios modelEnum.mustache

accidentally typescript-angular is fixed without template changes, but many others need their own fixes on template.

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh

  Commit all changed files. This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master. These must match the expectations made by your contribution. You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*. For Windows users, please run the script in Git BASH.
• File the PR against the correct branch: master, 5.1.x, 6.0.x
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

cc @TiFu (2017/07) @taxpon (2017/07) @sebastianhaas (2017/07) @kenisteward (2017/07) @Vrolijkx (2017/09) @macjohnny (2018/01) @topce (2018/10) @akehir (2019/07) @petejohansonxo (2019/11) @amakhrov (2020/02)