Limited string templates: $identifier

[RZhang05]

Working towards #3579

Description

This PR introduces a simple string template in the form of the character $ followed by an identifier ("$foo"), with plans to support the syntax of $ followed by an expression in curly braces later.

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• Targeted PR against feature/string-templates branch
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work
• Code follows the standards mentioned here
• Updated relevant documentation
• Re-reviewed Files changed in the Github PR explorer
• Added appropriate labels

[RZhang05]

Opened for early feedback.

• Need to add more tests across all stages of the feature
• Implementation questions marked with STRINGTODO in the code
• Further consider handling (or lack of handling) of error cases in the lexer and parser