Describe the enhancement
It would be great to have some function or expression to read file content (readFile or contents).
Something like {{ contents('.nvmrc') }}

Code Snippet

uses: setup-node@v1
with:
  node-version: {{ contents('.nvmrc') }}

Additional information
We already have functions that works with FileSystem, for example hashFiles.
Introducing contents command will bring benefits to a bunch of 1st party actions.
A few examples:

• very popular request to add support for parsing .nvmrc in setup-node: Support .nvmrc setup-node#32
• add support for .node-version in setup-node: Feature request: Support .node-version to specify the version setup-node#206
• support for reading .java-version file in setup-java: .java-version file support setup-java#113

It doesn't make much sense to implement this logic in every action. It would be great to have ability to read files via simple expression:

uses: setup-java@v1
with:
  java-version: {{ contents('.java-version') }}

uses: setup-node@v1
with:
  node-version: {{ contents('.nvmrc') }}