Update String's definitions

[sampersand]

IMPORTANT: The gsub! function doesn't have tests (but has been tested manually) because of a bug in the test harness

Test harness updates:

• TypeAssertions::RUBY_EXE added RUBY_EXE, which is the path to the ruby interpreter—Ruby's the executable guaranteed to be installed on any system we're testing (eg for Kernel#system)
• TypeAssertions#pass: Added pass, which is used in a few functions to indicate a test passes without using the type-checking assertions directly.

String updates (bolds are significant changes imo):

• String#{*,+,==,byteindex,center,concat,include?,{r,}index,{l,r}just,match{,?},{r,}partition,scan,sum,to_i,upto}: Updated variable names

• String#{capitalize{,!},chop!}: Added a TODO to add a variant for when arbitrary constraints come out.

• String#{bytes,chars,codepoints}: Returns self in the block variant

• String#{capitalize,downcase}!: All variants now return self?

• String#{grapheme_clusters,lines}: Added the block variant

• String#{clear,insert,prepend,replace,succ}: Returns self now

• String::selector: an alias for string used to indicate methods take a string whose contents have a special meaning

• String#{count,{delete,squeeze,tr{,_s}}{,!}}: Now uses String::selector, updated variable names

• String.try_convert: Added (String) -> String | (_ToStr) -> String | (untyped) -> nil

• String#initialize{,_copy}: Moved to the top of the class; initialize's return type was updated.

• String#%: The variants can be implicit

• String#{+,-}@: Return self | instance now

• String#<<: Separated variants out for the variable names

• String#<=>: Annotated the previously untyped variant, now untyped returns nil.

• String#===: Now an alias for String#==

• String#=~: Added the variant for when the other is not a Regexp but defines =~

• String#[]{,=}: Collapsed variants, added implicits, the regexp variants now uses MatchData::capture

• String#byteslice: Collapsed Range variants, and made implicit

• String#bytesplice: Added updates from Ruby 3.3

• String#casecmp{,?}: Split out string vs untyped variants; string variant in casecmp variant now returns (-1 | 0 | 1)?

• String#chomp: Separator can now be nil

• String#chomp!: Now returns self?, and nil separator always returns nil.

• String#delete_{prefix,suffix}!: Now returns self? not String?

• String#each_{byte,char,codepoint,grapheme_cluster,line}: Enumerator variant is now first, to be consistent with other core lib definitions.

• String#encode{,!}: All keyword args are now exhaustively annotated. The technically correct **untyped is not supplied.

• String::_EncodeFallbackAref: Added for use with String#encode{,!}

• String#force_encoding: Now uses encoding

• String#gsub{,!}: The Hash variant is now implicit, with a _ToS value; Enumerator is now above the block variant; !! gsub! isn't testable because of a bug in the test harness !!

• String#next{,!}: Now aliases for String#succ{,!}

• String#scrub: Added ?nil to the block variant

• String#setbyte: Now returns second variable, which is now _ToInt.

• String#slice!: Updated variable names, uses MatchData:capture, added implicits.

• String#split: Added nil value for pattern

• String#sub{,!}: The Hash variant is now implicit, with a _ToS value; self is now returned in the block form

• String#to_s: Now returns self | String

• String#to_str: An alias for String#to_s

• String#to_sym: An alias for String#intern

• String#unicode_normalize: Can now return self, named the variable

• String#unicode_normalize!: Always returns self, named the variable

• String#unpack: Implcits, and added the block variant

• String#unpack1: Implcits, added ?offset: int parameter

[sampersand]

<todo, clean this up, collapse commits down>
done

[sampersand]

This PR is waiting for #1643 to be merged

[soutaro]

self | instance type doesn't make much sense.
It will be a String instance or it's subclass, so self is good here.

[ParadoxV5]

Did you mean? instance

[soutaro]

Combine the two overloads for the case passing a value of String | Integer.
(This can be improved with future version of Steep, but currently it doesn't work for this.)

[soutaro]

As we discussed at Discord?

[soutaro]

For the case we pass String?.

[soutaro]

There may be a case that returns exact String, not self?

[soutaro]

Also replaced self types in tests with String.