DocumentStore refactor

[Techatrix]

Previously DocumentStore used a reference counting like approach for cleaning up unused document. This approach is unfitting because of cyclic dependencies.
As an example if A.zig imports B.zig and B.zig imports A.zig they will never be closed.
This new approach should collect all unneeded documents for cleanup.

CImports also received some care and are now managed globally instead of per document.

[leecannon]

This seems to have broken completion of @import("builtin").

Completion will show public decls from build.zig and ctrl+clicking will also open build.zig.

[Techatrix]

This seems to have broken completion of @import("builtin").

good catch! this is fixed now

Possible performance impacts.

[leecannon]

There is a significant performance hit associated with these changes.

TLDR: ReleaseSafe 73x longer to reach first textDocument/semanticTokens/full and 7x more memory in use

Process:

1. Clone one of the bigger projects zigwin32
2. Modify ZLS by changing Server.zig line 2410 (master) / line 2439 (PR) to log.info so the time to do is also displayed in release modes
3. open vscode in the zigwin32 project with no documents open
4. open win32.zig in the root of the project
5. wait for the time to do textDocument/semanticTokens/full to be printed

Results:

Source	Build Mode	Time	Memory	Time change from Master	Memory change from Master
Master	Debug	1,885ms	178mb	-	-
PR	Debug	49,170ms	1,139mb	~26x longer	~6x more
Master	ReleaseSafe	459ms	150mb	-	-
PR	ReleaseSafe	33,653ms	1,057mb	~73x longer	~7x more

I know this is only the initial hit when fist opening a project, but 33 seconds and a gig of ram is alot.

[Techatrix]

I didn't expect the performance penalty to be this bad. I will just continue to work on returning to "load on demand" when i have the time and then this could be merged.

[Techatrix]

The memory consumption and runtime should now be close to master.

[leecannon]

Looks good. 👍