There were scenarios in which patching a document would result in loss
of style for the template runs and, possibly, their right-adjacent run
as well (post-splitting). That was due to the run style elements not
being added to the newly created runs.

This commit addresses this issue by introducing a new, optional, flag
for the `patchDocument` function: `keepOriginalStyles`. It defaults to
`false` (current behavior) and, when `true`, ensures that there is no
loss of styling.

This should address dolanmiu#2293.