reserve comments for templates

[timqian]

issue: https://app.asana.com/0/1200011502754281/1200078207478206

This PR helps to reserve the comments in the serverless.yml of templates

Before

sls init eggjs-starter

After

[timqian]

The reason we lost the comments of serverless.yml after sls init a template is because we need to change the app key in the yml. Currently, we firstly turn this yml to an object, update this object and then recreate a yml. In this process, the comments are lost.

Unfortunately, js-yaml is not able to persist comments while parsing yml: nodeca/js-yaml#549

There are two ways to fix this issue.

Option 1:

Use libs that preserve comments.

Disadvatages

• There aren't many libs capable of doing this. After some search we find this lib: yawn-yaml
• this lib is not very actively maintained, we are using js-yml in other places and replacing this lib may introduce other issues.
• By using the lib above need to add at least 500kb to the components CLI
  • ref: https://github.com/mohsen1/yawn-yaml/blob/master/package.json
  • https://www.npmjs.com/package/yawn-yaml
  • https://github.com/connec/yaml-js/blob/master/yaml.js
  • https://www.npmjs.com/package/lodash

Option 2:

Use RegExp to replace app(current implementation)

I have tested this on the following 4 situations, it works well

• Template yml with comments
• Template yml without comments
• Rename app when sls init, e.g. sls init eggjs-starter --name hello
• Use default app name when sls init