CompileError should retain span information

[TedDriggs]

The current CompileError enum only accepts strings, so it cannot preserve spans that were available during AST parsing. This means errors are less precise than they could be; they highlight the entire struct, rather than the relevant meta-item.

[grv07]

Hello @TedDriggs , Please share some examples :) .

Thanks,

[TedDriggs]

You can compare errors from this branch to the ones in main: https://github.com/djc/askama/pull/537; try changing ext to ex in a test to see the difference.