Rename TextEdit getters and setters to match property names

[madmiraal]

Part of #16863, and the TextEdit sister PR to the LineEdit PR #47448.

Similar to LineEdit, TextEdit's caret_* properties getters and setters are also misnamed. This PR renames those getters and setters to match the property names. In addition, for consistency, it renames the other caret methods, signals, the editor settings and the documentation to refer to caret instead of cursor too.

Methods renamed:

Property	Old Method Name	New Method Name
	center_viewport_to_cursor	center_viewport_at_caret
	cursor_get_column	get_caret_column
	cursor_get_line	get_caret_line
	cursor_set_column	set_caret_column
	cursor_set_line	set_caret_line
	get_word_under_cursor	get_word_at_caret
	insert_text_at_cursor	insert_text_at_caret
caret_blink	cursor_set_blink_enabled	set_caret_blink_enabled
"	cursor_get_blink_enabled	is_caret_blink_enabled
caret_blink_speed	cursor_set_blink_speed	set_caret_blink_speed
"	cursor_get_blink_speed	get_caret_blink_speed
caret_block_mode	cursor_set_block_mode	set_caret_block_mode_enabled
"	cursor_is_block_mode	is_caret_block_mode_enabled
caret_mid_grapheme	set_mid_grapheme_caret_enabled	set_caret_mid_grapheme_enabled
"	get_mid_grapheme_caret_enabled	is_caret_mid_grapheme_enabled
caret_moving_by_right_click -> caret_moves_with_right_click	set_right_click_moves_caret	set_caret_moves_with_right_click_enabled
"	is_right_click_moving_caret	is_caret_moves_with_right_click_enabled

Old Signal	New Signal
cursor_changed	caret_changed

Editor Settings renamed:

Old Setting	New Setting
text_editor/cursor/scroll_past_end_of_file	text_editor/caret/scroll_past_end_of_file
text_editor/cursor/block_caret	text_editor/caret/block_mode_enabled
text_editor/cursor/caret_blink	text_editor/caret/blink_enabled
text_editor/cursor/caret_blink_speed	text_editor/caret/blink_speed

Finally, the property readonly has also been renamed to editable to make it consistent with LineEdit. I've kept this as a separate commit, because it also reverses the functionality.

[madmiraal]

Rebased following merge of #47625 and #47689.

[madmiraal]

Rebased following merge of #47448.

[akien-mga]

Superseded by #50371.