Add root: true to eslintrc

[hubgit]

Set root: true in the .eslintrc template, so that ESLint doesn't try to combine the configuration in the functions directory with the configuration in an .eslintrc file in the parent app directory.

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[hubgit]

@googlebot I signed it!

[samtstern]

@hubgit thanks for the contribution! Assigning to @bkendall who is our lint master.

[bkendall]

Sorry for being super delayed. I like this idea, but could you update this branch with master and then update both _eslintrc files (which were just updated to be a bit more simple) to include this change? Thanks!

[bkendall]

Taking the idea and running with it (keeping your initial commit) in #3029. Thanks!