Add support for ERB syntax highlighting in Heredoc sections

[domingo2000]

Motivation

Closes #2863

Implementation

This is a follow up of this PR #2903

Added .erb grammar to the vscode grammar extension.

Automated Tests

Added test for erb grammar in the grammars.test.ts

Manual Tests

I run the extension locally. The grammar is taking the ERB block in the heredoc. But for some reason, reusing the text.html.erb grammar of the RubyLSP extension gives different syntax highlighting than in a example.erb file.

example.rb

example.erb

[graphite-app]

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