feat: plugin install iteration 2

[Two-Hearts]

This PR is the 2nd iteration of plugin install.
Installation SHOULD pass in any of the following scenario:

1. Installing a single valid plugin executable file on file system. An example can be notation-com.example.plugin on Unix and notation-com.example.plugin.exe on Windows.
2. Installing from a dir which contains one and only one executable file with file name in the notation-{plugin-name} format.
3. On Unix, install from a dir which contains no executable file, however, there's one and only one candidate file with file name in the notation-{plugin-name} format. (Notation would set it to executable and try install it with a warning alert to the user). On Windows, install would fail in this case since '.exe' file extension is required.

Corresponding tests are added as well.

[codecov-commenter]

Codecov Report

Attention: 7 lines in your changes are missing coverage. Please review.

Comparison is base (706eab8) 73.34% compared to head (3d8621d) 73.35%.

Files	Patch %	Lines
plugin/manager.go	84.00%	3 Missing and 1 partial ⚠️
plugin/manager_unix.go	75.00%	2 Missing and 1 partial ⚠️

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Additional details and impacted files

@@           Coverage Diff           @@
##             main     #369   +/-   ##
=======================================
  Coverage   73.34%   73.35%           
=======================================
  Files          27       27           
  Lines        2480     2492   +12     
=======================================
+ Hits         1819     1828    +9     
- Misses        522      524    +2     
- Partials      139      140    +1     

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[shizhMSFT]

LGTM

[rgnote]

LGTM

[shizhMSFT]

LGTM

[JeyJeyGao]

LGTM