I don't think anyone has published a formal grammar definition but it should be easy enough to create from the F1 Quick Syntax Checker, the user manuals or online command documentation.
Most commands are fairly simple with some repeated and optional parameters, the SERIN command is perhaps the most complicated with optional qualifiers or just qualifiers. A few are two choice ...
HIGH <pin> { , <pin> ... }
SERVO { \[ <varOrValue> \] , } <pin> , [ OFF | <varOrByteValue> ]
HI2CIN { \[ <varOrByteValue> \] , } { <varOrValue> , } ( <var> { , <var> } )
Perhaps the main diffficulty is where some parameters are not available for some PICAXE variants.