Currently, template expressions and statements have their location
recorded relative to the HTML element they are in, with no handle to
absolute location in a source file except for a line/column location.
However, the line/column location is also not entirely accurate, as it
points an entire semantic expression, and not necessarily the start of
an expression recorded by the expression parser.
To support record of the source code expressions originate from, add a
new `sourceSpan` field to `ASTWithSource` that records the absolute byte
offset of an expression within a source code.
Implement part 2 of [refactoring template parsing for
stability](https://hackmd.io/@X3ECPVy-RCuVfba-pnvIpw/BkDUxaW84/%2FMA1oxh6jRXqSmZBcLfYdyw?type=book).
PR Close#31391
f65db20c6d