fix: underscore escaping for dotfiles (#1737)

* WIP: fix undersocre escaping for dotfiles 

(fix #1732)

* Fix charAt

* adding a short explanation to the docs.

* improved wording

docs/dev-guide/plugin-dev.md

@@ -235,6 +235,30 @@ export default {
 <%# END_REPLACE %>
 ```
 
+#### Filename edge cases
+
+If you want to render a template file that either begins with a dot (i.e. `.env`) you will have to follow a specific naming convention, since dotfiles are ignored when publishing your plugin to npm:
+```
+# dotfile templates have to use an underscore instead of the dot:
+
+/generator/template/_env
+
+# When calling api.render('./template'), this will be rendered in the project folder as:
+
+.env
+```
+Consequently, this means that you also have to follow a special naming convention if you want to render file whose name actually begins with an underscore:
+```
+# such templates have to use two underscores instead of the dot:
+
+/generator/template/__variables.scss
+
+# When calling api.render('./template'), this will be rendered in the project folder as:
+
+_variables.scss
+```
+
+
 ### Prompts
 
 #### Prompts for Built-in Plugins

packages/@vue/cli/lib/GeneratorAPI.js

@@ -138,9 +138,12 @@ class GeneratorAPI {
  let filename = path.basename(rawPath)
  // dotfiles are ignored when published to npm, therefore in templates
  // we need to use underscore instead (e.g. "_gitignore")
- if (filename.charAt(0) === '_') {
+ if (filename.charAt(0) === '_' && filename.charAt(1) !== '_') {
  filename = `.${filename.slice(1)}`
  }
+ if (filename.charAt(0) === '_' && filename.charAt(1) === '_') {
+ filename = `${filename.slice(1)}`
+ }
  const targetPath = path.join(path.dirname(rawPath), filename)
  const sourcePath = path.resolve(source, rawPath)
  const content = renderFile(sourcePath, data, ejsOptions)