Regression for let in allowing variables to start with _ (underscore)

[cipriancraciun]

Up to version 12 (excluding), one could have written {% let _x = 7 %} and have it correctly compile. In version 12.1, it fails with:

error: literals are not allowed on the left-hand side of an assignment

(According to the Rust reference, identifiers are allowed to start with an _, as per https://doc.rust-lang.org/reference/identifiers.html.)

It appears that neither identifiers that start with an upper-case letter (like X) are allowed, with the same error.

[djc]

Sorry for the regression, looks like this has been fixed on main though -- see #1035 for the upcoming 0.13 release.