New file picker.

[gcoakes]

Request

I find it tedious to switch between helix and the terminal to create new directories and files. Add a new picker menu which shows only directories in the list, but opens a new file at the location when you select. For example, entering this picker while within the helix repo with an empty search would yield this:

|
> ./
  ./.cargo/
  ./.github/
  ./book/
  ./contrib/
  ./docs/
  ./helix-core/
  ./helix-dap/
  ./helix-lsp/
  ./helix-syntax/
  ./helix-term/
  ./helix-tui/
  ./helix-view/
  ./runtime/
  ./xtask/

where the | is text entry box and the > is the cursor.

While entering a name without a path separator at the end, it would filter the directory names:

| helix
> ./helix-core/
  ./helix-dap/
  ./helix-lsp/
  ./helix-syntax/
  ./helix-term/
  ./helix-tui/
  ./helix-view/

After entering a directory name with a path separator, it would look like this:

| helix-term/
> ./helix-term/
  ./helix-term/src

Pressing enter while in this state would make any parent directories and open the file:

| helix-term/src/foo/mod.rs
> ./helix-term/src/

Pressing enter while in this state would populate the picker text entry box with the selection under the cursor:

| 
  ./
  ./.cargo/
  ./.github/
  ./book/
  ./contrib/
  ./docs/
  ./helix-core/
  ./helix-dap/
  ./helix-lsp/
  ./helix-syntax/
> ./helix-term/
  ./helix-tui/
  ./helix-view/
  ./runtime/
  ./xtask/

would yield:

| helix-term/
> ./helix-term/
  ./helix-term/src

The default config would open with the parent of the current file already selected. i.e.: if you enter the picker while editing helix-term/src/ui/picker.rs, then the picker's initial state would look like this:

| helix-term/src/ui/
> helix-term/src/ui/

Implementation

I've scoped this out a bit but don't feel confident enough in my understanding of the helix_term::ui::Picker struct to implement this. It looks like Picker only permits selection of matches, and does not allow the user to select a non-existing file for FilePicker. I think changes would need to be made to Picker to allow non-matching paths to be sent to the callback_fn. Possibly this could be implemented as an new option that defaults to false so as to not break current pickers.