Adopt a "practical" Semantic Versioning

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Feature description

WeeChat should adopt a "practical" Semantic Versioning.
"Practical" means some API "minor" breaking changes are allowed without incrementing the major version number.

This could start with version 3.9 renamed to 4.0.0, as major breaking changes are introduced:

• build with autotools removed (Remove build with autotools #1860)
• case sensitive identifiers (Make most WeeChat identifiers case sensitive #1872, Autocomplete wrong case #398, bug #32213)
• key bindings improvements (Force Control keys to lower case (ctrl-a instead of ctrl-A) #1875, Add aliases for key bindings #1238).

Specification: https://specs.weechat.org/specs/2023-003-practical-semantic-versioning.html