feat(expr): add regexp_count

[xzhseh]

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What's changed and what's your intention?

resolve #11735

Checklist

• I have written necessary rustdoc comments
• I have added necessary unit tests and integration tests
• I have added fuzzing tests or opened an issue to track them. (Optional, recommended for new SQL features Sqlsmith: Sql feature generation #7934).
• My PR contains breaking changes. (If it deprecates some features, please create a tracking issue to remove them in the future).
• All checks passed in ./risedev check (or alias, ./risedev c)
• My PR changes performance-critical code. (Please run macro/micro-benchmarks and show the results.)

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Documentation

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Release note

support regexp_count

[github-actions]

license-eye has totally checked 4020 files.

Valid	Invalid	Ignored	Fixed
1771	1	2248	0

Click to see the invalid file list

• src/expr/src/expr/expr_regexp_count.rs

[xzhseh]

Syntax of regexp_count is as: regexp_count(string, pattern [, start [, flags ]]).
Now the supported flags option is ic, global flag is implicitly supported and could not be specified.

[wangrunji0408]

Generally LGTM.

[codecov]

Codecov Report

Merging #11975 (139fe7b) into main (5913af4) will decrease coverage by 0.04%.
The diff coverage is 0.82%.

@@            Coverage Diff             @@
##             main   #11975      +/-   ##
==========================================
- Coverage   69.90%   69.86%   -0.04%     
==========================================
  Files        1389     1390       +1     
  Lines      232946   233066     +120     
==========================================
- Hits       162840   162838       -2     
- Misses      70106    70228     +122     

Flag	Coverage Δ
rust	69.86% <0.82%> (-0.04%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
src/expr/src/expr/build.rs	78.43% <0.00%> (-0.52%)	⬇️
src/expr/src/expr/expr_regexp.rs	0.00% <ø> (ø)
src/expr/src/expr/expr_regexp_count.rs	0.00% <0.00%> (ø)
src/expr/src/expr/mod.rs	62.96% <ø> (ø)
src/frontend/src/expr/pure.rs	87.69% <ø> (ø)
src/frontend/src/expr/type_inference/func.rs	82.13% <0.00%> (-0.20%)	⬇️
src/frontend/src/binder/expr/function.rs	78.98% <100.00%> (+0.01%)	⬆️

... and 3 files with indirect coverage changes

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[xzhseh]

cc @TennyZhuang @wangrunji0408, I'm ready to merge.

[wangrunji0408]

LGTM! Good work.