feat: set cwd for dlv based on program

[mcoqzeug]

This potentially resolves #85.

• Similar to vscode-go, set dlv.executable.cwd based on the value of debug_config.program. If program is a directory, set cwd to program. If program is a path to a go file, set cwd to its parent directory.
• Convert program to absolute path.

We probably don't need config.delve.cwd anymore.

Reference: https://github.com/golang/vscode-go/blob/master/extension/src/goDebugConfiguration.ts#L545

[leoluz]

@mcoqzeug Thank you for removing plenary dependency.
I noticed that you are re-implementing file path logic that is already available as vim functions. The get_parent function for example can be replaced by fnamemodify (see https://stackoverflow.com/questions/16485748/how-to-get-the-parent-directory-of-a-path-string). Similar strategy can be used to extract the file extension with filename-modifier :e (see https://vimhelp.org/cmdline.txt.html#filename-modifiers)

[TendonT52]

Thank you, I'm just using this branch, and it works. Hope this will merge soon.