deprecate the *_exist family of functions

[aylapear]

Deprecate these functions:

• 'sbf_block_exists'
• 'sbf_data_exists'
• 'sbf_number_exists'
• 'sbf_object_exists'
• 'sbf_plot_exists'
• 'sbf_string_exists'
• 'sbf_table_exists'

Only sbf_object_exists() is being used in a couple scripts so we have decided to deprecate these functions.

We will be at the first stage of deprecation, use this to help guide which stage/level of deprecation https://lifecycle.r-lib.org/reference/deprecate_soft.html . There are examples in the chk package in the R/deprecated.R file.

Since all the exists functions are in their own script just do the deprecation there (ie no need to move to a deprecated.R script)

[joethorley]

done by Alan