SPICE-0019: Allow trailing commas in comma-separated syntax elements

[HT154]

Implements apple/pkl-evolution#21

This changes Pkl's grammar but is not breaking. It only admits inputs that were previously invalid.

Covers these syntax elements:

• Function parameter lists (method definitions) - tested in parser/methodTrailingCommas.pkl
• Argument lists (method calls) - tested in parser/methodTrailingCommas.pkl
• Type argument lists (in declared type names) - tested in parser/trailingCommas.pkl
• Type parameter lists (in class/module headers) - tested in parser/trailingCommas.pkl
• Type constraint lists - tested in parser/constraintsTrailingComma.pkl
• Function type parameter lists (function type annotations) - tested in parser/lambdaTrailingCommas.pkl
• Object body parameter lists (sugar'd lambdas) - tested in parser/lambdaTrailingCommas.pkl

Resolves #1132

[HT154]

This extra check is needed because some callers parse the element of the list themselves (while determining what construct it is) so it's possible for this function to be reached while at terminator.

[bioball]

Just a few nits, but overall this looks good!

Would also like @stackoverflow to take a look and review the parser changes.

Do you also mind writing a quick SPICE documenting the changes here?

[stackoverflow]

Looks good!

[bioball]

LGTM! This is a nice improvement, thanks!