Impl displaying signature from LSP

[MinusGix]

• Added an entry to CHANGELOG.md if this change could be valuable to users

This PR adds support for displaying the signature of a function when it is being called, by asking the language server for the information.
It binds this to Ctrl+Shift+Space, since that is a common binding for showing it when it is closed.

Notes:

• I put the get_signature keybinding in both keymaps-macos.toml and keymaps-nonmacos.toml, but they are the same. I did that due to get_completion being defined in both, but is that really intended? Shouldn't it just be in keymaps-common.toml?
• This doesn't yet implement switching between signatures (like in overloads), but I believe it should change the selection based on what LSP tells us; which covers many of the cases of using it. It would be good to support that eventually.

[codecov-commenter]

Codecov Report

Merging #1720 (509007c) into master (e65d64b) will decrease coverage by 0.08%.
The diff coverage is 0.00%.

@@            Coverage Diff            @@
##           master   #1720      +/-   ##
=========================================
- Coverage    6.64%   6.56%   -0.09%     
=========================================
  Files         126     127       +1     
  Lines       53284   53944     +660     
=========================================
  Hits         3541    3541              
- Misses      49743   50403     +660     

Impacted Files	Coverage Δ
lapce-core/src/command.rs	0.00% <ø> (ø)
lapce-data/src/command.rs	0.00% <ø> (ø)
lapce-data/src/config.rs	0.00% <ø> (ø)
lapce-data/src/data.rs	0.00% <0.00%> (ø)
lapce-data/src/editor.rs	0.00% <0.00%> (ø)
lapce-data/src/markdown/mod.rs	0.00% <0.00%> (ø)
lapce-data/src/proxy.rs	0.00% <0.00%> (ø)
lapce-data/src/signature.rs	0.00% <0.00%> (ø)
lapce-proxy/src/dispatch.rs	0.00% <0.00%> (ø)
lapce-proxy/src/plugin/lsp.rs	0.00% <0.00%> (ø)
... and 10 more

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