environment: move ignore_case into repo_config_values

environment: move ignore_case into repo_config_values

The 'core.ignorecase' configuration which is stored as the
global variable 'ignore_case' acts as a core filesystem
capability flag.

Move this global variable into 'struct repo_config_values' to tie it
to the specific repository instance it was read from. This reduces
global state and aligns with the ongoing libification effort.

To ensure code readability, the getter function
'repo_ignore_case()' is introduced.

Mentored-by: Christian Couder <redacted>
Mentored-by: Ayush Chandekar <redacted>
Mentored-by: Olamide Caleb Bello <redacted>
Signed-off-by: Tian Yuchen <redacted>
Signed-off-by: Junio C Hamano <redacted>