Fix: Variable name can start with a digit

[OverflowCat]

According to the spec, an <ident-token> cannot start with a digit. However, for variables that begin with a double-hyphen --, the name following it CAN start with a digit.

For example, --7-grid-columns-minus-last-gap (found by @yzqzss in GitHub's landing page) is a completely valid variable name.

Current grammar does not properly highlight it:

This change to the lexical grammar was made in 2014, although they didn't mention it clearly:

Change the definition of ident-like tokens to allow "--" to start an ident. As part of this, rearrange the ordering of the clauses in the "-" step of consume a token so that s are recognized as such instead of becoming a -- .

The old definition:

[OverflowCat]

@microsoft-github-policy-service agree

[romainmenke]

Thank you for the detailed description and fix!
LGTM

[yzqzss]

@microsoft-github-policy-service agree