Creation of a Selector configuration option for templates

[lalvarezt]

The purpose of the selector is to formalize a way to render the templates when multiple items are selected
Note: this requires the HEAD version from string_pipeline that has the structured template feature

Let's consider the following files channel as the syntax definition

[metadata]
name = "files"
description = "A channel to select files and directories"
requirements = ["fd", "bat"]

[ui]
results_max_selections = 3 # Maximum allowed selections (0 = disabled)

[ui.preview_panel]
size = 40

[source]
command = ["fd -t f", "fd -t f -H"]
# output = "echo {}"  # legacy notation still supported

# mode definition
# single: only the first item (if more than one) is passed to the template formatter
# concatenate: all selected items are passed to the template formatter using format_with_input, the result is then concatenaded with separator and optionally escaped
# one-to-one: all selected items are passed to the template formatter using format_with_input, the result is then assigned to the corresponding position in the template and optionally escaped
# see below for examples

[source.output]
template = "echo {}"
mode = "concatenate"  # one of [single, concatenate, one_to_one]
separator = " "
shell_escaping = true

[ui.preview_panel.header]
template = "{}"
mode = "concatenate"   # one of [single, concatenate, one-to-one]
separator = " - "
shell_escaping = false

[ui.preview_panel.footer]
template = "1: {} - 2: {} - 3: {}" # notice max_selections is 3 above
mode = "one-to-one"
separator = ""                     # makes no sense in this mode, ignored
shell_escaping = false

[preview.command]
template = "bat -n --color=always {}"
mode = "single"
separator = ""                        # makes no sense in this mode, ignored
shell_escaping = true
env = { BAT_THEME = "ansi" }

[keybindings]
shortcut = "f1"
f12 = "actions:edit"

[actions.edit]
description = "Opens the selected entries with the default editor (falls back to vim)"
# use `mode = "fork"` if you want to return to tv afterwards
mode = "execute"

[actions.edit.command]
template = "${EDITOR:-vim} {}"
mode = "single"
separator = ""                 # makes no sense in this mode, ignored
shell_escaping = true

with the following entries

file1.txt
file 2.txt
file3.txt
file4.txt

when opening television it will look like this, considering the first entry as hovered

preview_panel.header = "file1.txt"
preview_panel.footer = "WARNING: Empty placeholders detected (1 entry for 3 placeholders)"
preview.command = "bat -n --color=always file1.txt"
source.output = "echo file1.txt"

selecting 3 items will look like this

preview_panel.header = "file1.txt - file 2.txt - file3.txt"
preview_panel.footer = "1: file1.txt - 2: file 2.txt - 3: file3.txt"
preview.command = "bat -n --color=always file1.txt"
source.output = "echo file1.txt 'file 2.txt' file3.txt"

@alexpasmantier wdyt?

EDIT: add missing source.output
EDIT2: moved results_max_selections into ui section
EDIT3: add note about legacy notation still supported

[lalvarezt]

POC in #709