[Feature Request]: fixed layout for flextable

### Feature description

Currently, exporting gtsummary objects relies on flextable. User frequently want to set manually the width of the column, however, upon export of the flextable to docx, this manually selected column width is not respected. To solve this issue, users have to manually specify that the layout of the flextable is "fixed" (`flextable::set_table_properties(layout = "fixed")`). Could we intergrate that bit to the roche theme so that flexable object comming from gtsummary object have a fixed layout by default

e.g.

```r
fixed_theme <- list(
# This runs code on the flextable object immediately after conversion
"pkgwide-fun:pre_conversion" = function(x) {
if (inherits(x, "flextable")) {
x %>%
set_table_properties(layout = "fixed") %>%
width(j = 1, width = 2.5, unit = "in") # Always make first column 2.5 inches
} else {
x
}
}
)

set_gtsummary_theme(fixed_theme )
```

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