line_consumption: Fix coding style and types

- Format code acording to style guide
- Change return types and types used in functions
line_consumption: Fix clint errors

* Fix clint errors that were the results of fixes after adding
  line_consumption.c to the list of files compiled with -Wconversion
line_consumption: Fix conversion error on 32bit machines

* After changing a variable type from int to `int64_t` compilation failed on
  32bit architectures due to a conversion warning. Changing it to `int32_t`
  makes the build fail on 64bit architectures.
line_consumption: Fix typos

Problem:
  - Some comments contained typos

Solution:
  - Use spell checking to find those typos and correct them
line_consumption: Fix file comment

Problem:
  - The comment describing the file contents is not using the
    doxygen @file notation
  - File comment is below includes but should be above

Solution:
  - Make use of the @file directive
  - Move comment to the top of the file
line_consumption: Remove unnecessary casts

Problems:
  - The type change of one of the parameters in `plines_win_col` makes some
    casts in the function body superfluous
  - The move of the comment about the file contents left some unnecessary
    newlines

Solutions:
  - Remove unnecessary casts in function body of `plines_win_col` to `colnr_T`
  - Remove unnecessary empty lines
line_consumption: Remove unnecessary check for `true`

Problems:
  - `lineFolded` in `src/nvim/fold.c` has a wrong comment claiming
    that it can return the value `MAYBE`, the implementation doesn't
    allow the return of `MAYBE` though
  - An if check in `plines_win_nofill` in `src/nvim/line_consumption`
    checks the return value of `line_folded` for `true` because of
    the mentioned comment.

Solutions:
  - Fix comment of `lineFolded` in `src/nvim/fold.c`
  - Remove unnecessary check for return value of `lineFolded` in
    `plines_win_nofill` of `src/nvim/line_consumption.c`