206 sourcing a file will lose the current path value

[prsabahrami]

Fixes #206.
I also updated the behaviour of exit command in subshells.
Before this PR:

(exit)

would exit the shell.
However, this is not expected behaviour and is fixed in this PR.

[codecov]

Codecov Report

Attention: Patch coverage is 30.00000% with 42 lines in your changes missing coverage. Please review.

Project coverage is 63.89%. Comparing base (1405935) to head (6505739).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/deno_task_shell/src/shell/execute.rs	42.85%	20 Missing ⚠️
crates/deno_task_shell/src/shell/types.rs	12.50%	7 Missing ⚠️
crates/shell/src/execute.rs	0.00%	7 Missing ⚠️
crates/shell/src/main.rs	0.00%	7 Missing ⚠️
crates/shell/src/commands/set.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #248      +/-   ##
==========================================
- Coverage   64.06%   63.89%   -0.18%     
==========================================
  Files          32       32              
  Lines        3648     3664      +16     
==========================================
+ Hits         2337     2341       +4     
- Misses       1311     1323      +12     

Files with missing lines	Coverage Δ
crates/deno_task_shell/src/shell/commands/exit.rs	100.00% <100.00%> (ø)
crates/shell/src/commands/set.rs	83.33% <0.00%> (ø)
crates/deno_task_shell/src/shell/types.rs	51.09% <12.50%> (-0.38%)	⬇️
crates/shell/src/execute.rs	0.00% <0.00%> (ø)
crates/shell/src/main.rs	0.00% <0.00%> (ø)
crates/deno_task_shell/src/shell/execute.rs	75.78% <42.85%> (-0.44%)	⬇️

... and 1 file with indirect coverage changes