feat: Compare existing text case-insensitive when inserting completion without textedit

[FlorianKroiss]

Consider this piece of code:

MY_SUPER_DUPER_CONSTANT = 42

print(my_super|)

With | being the cursor, the server proposes a completion with MY_SUPER_DUPER_CONSTANT.
If we accept this proposal, we get the following result, because we compare the existing characters case-sensitive, when trying to figure out where to place the completion and what characters we can re-use:

print(my_superMY_SUPER_DUPER_CONSTANT)

With this PR, we do the comparison case-insensitive, which instead gives us:

print(MY_SUPER_DUPER_CONSTANT)

Note that this is only necessary for rudimentary completions which only have a label but no textedit.