[JS] Set embedded templates directory when generating Javascript code

[me717]

PR checklist

• Read the contribution guildelines.
• Ran the shell/batch script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates)
• Filed the PR against the correct branch: master for non-breaking changes and 2.3.0 branch for breaking (non-backward compatible) changes.

Description of the PR

Set embedded template directory when generating Javascript code
(details of the change, additional tests that have been done, reference to the issue for tracking, etc)

[wing328]

@me717 thanks for the PR. I found several generators missing the embeddedTemplateDir as well:

swagger-codegen|master⚡ ⇒ grep -R templateDir modules/swagger-codegen/src/main/java/io/swagger/codegen/languages/* | grep -v embeddedTemplateDir
modules/swagger-codegen/src/main/java/io/swagger/codegen/languages/BashClientCodegen.java:    templateDir = "bash";
modules/swagger-codegen/src/main/java/io/swagger/codegen/languages/CppRestClientCodegen.java:        templateDir = "cpprest";
modules/swagger-codegen/src/main/java/io/swagger/codegen/languages/GoClientCodegen.java:        templateDir = "go";
modules/swagger-codegen/src/main/java/io/swagger/codegen/languages/JMeterCodegen.java:    templateDir = "JMeter";
modules/swagger-codegen/src/main/java/io/swagger/codegen/languages/JavascriptClientCodegen.java:        templateDir = "Javascript";

I'll file a separate PR to clean those up.