feat: specify --init filename/path (#2018)

* feat: specify init filename with --taskfile flag

previously, it was not possible to specify which filename to use when initializing a new Taskfile as it was hardcoded as "Taskfile.yml".

now the --taskfile flag specifies where to write the file to, and the first * contained in it will be replaced by "Taskfile", so `task -it *.yaml` will create a `Taskfile.yaml` file.

* docs: update CLI reference

* fix Flags header being inside tip admonition
* change -t flag's default column and add a description
* add Default Filenames section

* docs: revert adding Default Filenames section

I didn't realize it already existed elsewhere.

* refactor: use path instead of filepath on InitTaskFile

as requested to prevent ambiguity with the stdlib package.

* fix TestInit (incorrectly merged)

* docs: remove outdated info on --taskfile flag

* refactor task initialization changes

- remove const DefaultTaskInitFilename from taskfile/taskfile.go
- revert description of Entrypoint flag
- make InitTaskfile accept a path to either a file or a directory, and join the default Taskfile name+ext to it if it is a directory
- take the target file path from the first argument instead of the Entrypoint flag
- detect extension-only filenames (".yaml") instead of replacing "*" with "Taskfile"
- use different format in success log so that it makes sense at different paths than the current dir

* print colon instead of "at"

it's a lot cleaner in most cases.

* rewrite init tests

test both initializing to a directory path and a file path

* return final path from InitTaskfile

...and print it's relative representation

* fix lint error (ineffassign)

* use filepathext.TryAbsToRel() instead

* define and use filepathext.IsExtOnly()

* link to default filenames list in cli ref docs

(specifically in the --taskfile flag description)

internal/filepathext/filepathext.go

@@ -55,3 +55,9 @@ func TryAbsToRel(abs string) string {
 
 	return rel
 }
+
+// IsExtOnly checks whether path points to a file with no name but with
+// an extension, i.e. ".yaml"
+func IsExtOnly(path string) bool {
+	return filepath.Base(path) == filepath.Ext(path)
+}