DiffScreen enhancements

[tanvirtin]

Closes: https://github.com/tanvirtin/vgit.nvim/issues/114

Users are given the ability to do the following:

• Stage all hunks in "diff_preview" and "hunk_preview" pressing "S"
• Unstage all hunks in "diff_preview" and "hunk_preview" pressing "U"
• Stage a hunk in "diff_preview" and "hunk_preview" pressing "s" on a hunk
• Unstage a hunk in "staged_diff_preview" and "hunk_staged_preview" pressing "u" on a hunk
• Switch between unstage and staged version of the view by pressing "t"

[nyngwang]

Many thanks!